Petrol/Diesel Level Sensor

This sensor is particularly suitable for use in small spaces, such as the petrol tank of a  motorbike. It has the advantage of not having any moving parts, unlike a conventional sensor with a float and float arm that make it difficult to fit in a tank.

The sensor circuit is made from standard, inexpensive components and can be put together for little money.
Petrol/Diesel Level Sensor Circuit Diagram :

The operating principle is  based on  measuring  the forward volt-ages of two identical diodes (check this  first by measuring  them).  The forward voltage of a diode decreases with increasing junction temperature. lf a resistor is placed close to one of the two diodes, it will be heated slightly if it extends above the surface of the  petrol. For best results,the other diode (used for reference) should be located at the same level. lf the diodes are covered by the petrol in the tank, the heating resistor will not have any effect because it will be cooled by the petrol. An opamp compares the voltage across the two diodes, with a slightly smaller current passing through the reference diode. When the petrol level drops, the output of the opamp goes high and the output transistor switches on. This causes a sense resistor to be connected in parallel with the sensor output. Several sensor circuits can be used together, each with its own switched sense resistor connected in parallel with the output, and the resulting output  signal can be used to drive a meter or the like.
Using this approach, the author built a petrol tank' sensors trip' tank consisting of five PCBs, each fitted with two sensor circuits. With this sensor strip installed at an angle in the tank, a resolution of approximately 1.5 litre per sensor is possible. Many tanks have an electrical fitting near the bottom for connection to a lamp on the instrument panel that indicates the reserve level. The sensor strip can be used in its place.
You will have to experiment a bit with the values of the sense resistors, but do not use values lower than around'100 O. It is also important to fit the diodes and heater resistor in a little tube with a small opening at the bottom so that splashing petrol does not cool the heater resistor, since this would result in false readings.
The circuit should be powered from a regulated supply voltage of 5 to 6 V to prevent the heating resistors from becoming too hot. After testing everything to be sure that it works properly, it's a good idea to coat the circuit board with epoxy glue to provide better protection against the petrol.
Tip: you can use the well-known 1M3914 to build a LED display with ten LEDs, which can serve as a level indicator. Several examples of suitable circuits can be found in back issues of Elektor.
Note: this sensor circuit is not suitable for use in conductive liquids.

Author : Paul de Ruijter - Copyright: Elektor

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Cat And Dog Repellent

The electronic dog repellent circuit diagram below is a high output ultrasonic transmitter which is primarily intended to act as a dog and cat repeller, which can be used individuals to act as a deterrent against some animals. It should NOT be relied upon as a defence against aggressive dogs but it may help distract them or encourage them to go away and do not consider this as an electronic pest repeller. The ultrasonic dog repellant uses a standard 555 timer IC1 set up as an oscillator using a single RC network to give a 40 kHz square wave with equal mark/space ratio.

This frequency is above the hearing threshold for humans but is known to be irritating frequency for dog and cats. Since the maximum current that a 555 timer can supply is 200mA an amplifier stage was required so a high-power H-bridge network was devised, formed by 4 transistors TR1 to TR4. A second timer IC2 forms a buffer amplifier that feeds one input of the H-bridge driver, with an inverted waveform to that of IC1 output being fed to the opposite input of the H-bridge.

Circuit diagram:

Cat And Dog Repellent Circuit Diagram

This means that conduction occurs through the complementary pairs of TR1/TR4 and TR2/TR3 on alternate marks and spaces, effectively doubling the voltage across the ultrasonic transducer, LS1. This is optimised to generate a high output at ultrasonic frequencies. This configuration was tested by decreasing the frequency of the oscillator to an audible level and replacing the ultrasonic transducer with a loudspeaker; the results were astounding. If the dog repellent circuit was fed by a bench power supply rather than a battery that restrict the available current, the output reached 110dB with 4A running through the speaker which is plenty loud enough!

The Dog and Cat repellant was activated using a normal open switch S1 to control the current consumption, but many forms of automatic switching could be used such as pressure sensitive mats, light beams or PIR sensors. Thus it could be utilise as part of a dog or cat deterrent system to help prevent unwanted damage to gardens or flowerbeds, or a battery powered version can be carried for portable use. Consider also using a lead-acid battery if desired, and a single chip version could be built using the 556 dual timer IC to save space and improve battery life.

Source : www.extremecircuits.net

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Simple 9-Way Cable Identifier

Here is a simple way of identifying multiple cables (with the aid of a multimeter). The circuit consists of a series of resistors, selected so that they give readings that coincide with the 1-9 numerals on the 10V scale on a multimeter switched to the Ohms x 100 range. In use, a common wire needs to be chosen and this is usually the shield wire.

The resistors go to one end of the cables to be identified, while the multimeter is used at the other end to check the values and identify each lead. Up to nine cables can be identified at a time. If a mistake is made in choosing the common lead, the readings will all be wide of the 1-9 numerals on the 10V scale, thus making the mis­take obvious.

Circuit diagram:

Simple 9-Way Cable Identifier

Source : http://www.extremecircuits.net

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Dimmer Control Voltage Polarity Changer Circuit

Some older Strand dimmer units used a zero to -10V control signal, and the standard analogue control voltage is zero to +10V. This project allows the easy conversion from one standard to another. This is a very simple project, but may turn out to be a lifesaver for small theatre groups and the like. It has come to my attention that there are still a great many old Strand dimmers very much in use. The problem is that they are just too reliable, and won't go away ... but, they use a zero to -10V control signal, so are incompatible with the dimmer unit in these project pages, and with any new commercial analogue control console.

In addition, there are no doubt quite a few old lighting consoles that use this standard, which means that they can't drive modern dimmer packs. As it turns out, a simple opamp inverter will convert either standard to the other. This is shown in Figure 1.

 Figure 1 - Dimmer Control Signal Inverter

There is really nothing to it. Use as many circuits as needed, and a simple power supply (such as that in Project 05) will drive as many of these inverters as are likely to be required in any lighting setup. The above circuit has two channels, and may be simply repeated as many times as you need to get the required number of channels. The 100 ohm resistors on each output are there to prevent the opamps from oscillating when supplying a capacitive load (such as a coax cable).
With an input of zero volts, the output will also be at zero volts. As the input increases (or decreases in the case of the -10V control) the output will change by exactly the same value, but in the opposite direction. Wiring is not critical, the 1458 opamps specified are very cheap (but more than capable of doing the job), and they can be built very simply on Veroboard or similar. Supplies should be bypassed to common (ground) with 10uF electrolytic caps.

source: http://sound.westhost.com/project90.htm

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Flip-Flop Using CMOS NAND Gates

Using just two NAND or inverter gates its possible to build a D type (or ‘toggle’) flip-flop with a push-button input. At power-up the output of gate N2 is at a logical ‘1’, ensuring that transistor T2 is switched off. When the push-button is pressed the output of N2 changes to a logical ‘0’ and transistor T2 conducts. The coupling between N1 and N2 ensures that the output of N1 will always be the inverse of N2 so T1 and T2 will always be in opposite states and will flip each time the push-button is pressed. In some cases it is possible to omit T1 and T2 and use the outputs of N2 and N1 to drive external circuitry directly but only if the loading on these outputs is low enough.

The 4000 series CMOS family can source/sink a maximum of 0.5mA (at 5 V) so for the sake of safety its best to use these open-collector configured transistor buffers. This circuit is particularly useful if you have some unused gates left over in a circuit design, avoiding the need to add a dedicated flip-flop IC. It is worth remembering that all other unused CMOS gates must have their inputs connected to either the positive or ground rail. The supply voltage can be in the range 3 V to 15 V for CMOS ICs and the current taken by this circuit is between 0.2 mA and 5 mA (no load).

Source : www.extremecircuits.net

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Electronic Die

The simplicity of a traditional die makes it exceptionally difficult to create a fully equivalent electronic version, if only because an electronic version requires a power supply and a collection of electronic components that occupy a much larger volume than a normal die. This article describes an electronic die that can be built using normal components or SMDs as desired, and which comes very close to having the same format as a traditional die in the latter case. Despite its simplicity, this electronic die incorporates several interesting features. For instance, the range of ‘spots’ can be increased from 1–6 to 0–9 using a jumper, and it has standby function that disables the display approximately 8 seconds after the die has been ‘thrown’, in order to save energy.

The electronic die also uses energy efficiently by driving the display in pulsed mode. As a result of the latter two features, the current consumption of the circuit is approximately 25 mA in use and 12 mA in standby. This means that it can easily be powered by a 9-V battery. The circuit consists of the following parts: a free-running oscillator (IC1a), additional logic for driving the display (IC1c & IC1d), a timer (IC1b), a counter (IC3) and a display decoder (IC2). The oscillator is very simple. Its frequency, which is determined by R1 and C1, is approximately 225 Hz, with a duty cycle of around 50–60 percent. The signal from the oscillator acts as a clock signal for the counter (via R2) and a blanking signal for the display decoder (via IC1d).

However, the counter will not count as long as the ‘throw’ switch (S1) remains closed, since the clock input of IC3 is grounded by S1. The blanking input of the display decoder is driven by a pulse waveform, so the display is in principle illuminated only around 50 percent of the time, but it appears to be constantly illuminated due to the high clock frequency. The standby mode works as follows. As long as there is a signal on the clock input of the counter (S1 pressed), the output of gate IC1b is low and the display is enabled. If S1 is released, the counter stops and a number will be shown on the display. However, the clock pulses will have charged C2 via D1, and C2 will slowly discharge via R4.

After approximately 8 seconds, the output of gate IC1b will go high, causing the display to be blanked. The design of the counter is relatively simple. It is wired as an up counter by connecting the U/D pin to VCC. The preset inputs (pins 4, 12, 13 and 3) are configured to binary ‘0001’, and the counter normally has a counting range of 0–9 (pin 9 connected to ground). Diodes D2, D3 and D4, in combination with resistor R5, act as a logic AND gate, so if the value of the counter is greater than 6, the preset value of 1 is latched into the counter and it starts to count again from 1 to 6. This only happens if jumper J1 is open. If it is closed, the preset pulse on PREN is suppressed and the counter range is 0–9. The A, B, C and D inputs of the decoder IC (IC2) are driven directly by the counter.

The series resistors normally used for the individual segments of the display are instead placed in the common-cathode lead (R7 & R8). This has the advantage of allowing the number of resistors to be reduced, although it has the drawback that the brightness of the display depends on the displayed number. If the segment current is sufficiently large, (light) saturation occurs and this brightness variation is no longer noticeable. The Blank input (BL) controls whether the display is enabled. If you choose to build this circuit using SMD technology, that will not affect the schematic diagram, but it will naturally affect the choice of components. In this case, SMD components must be used for the resistors and C1, the diodes must be replaced by BAS32 types, and BT versions of ICs IC1–IC3 must be used instead of conventional types.
An SMD version of C2 was not used in the prototype, since SMD electrolytic capacitors are expensive, and normally they are only sold in lots of 10, just like other passive components. It is also recommended to use a socket for the display of the SMD version of the electronic die, to allow the space under the display to also be used and the dimensions of the circuit board to be further reduced. Any desired DC power source providing a voltage of 5 to 15 V can be used as a power supply. Due to the low current consumption of the circuit, a 9-V battery will last quite a long time.

Source : www.extremecircuits.net

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PWM Modulator

If you ever thought of experimenting with pulse-width modulation, this circuit should get you started nicely. We’ve kept simplicity in mind and used a dual 555 timer, making the circuit a piece of cake. We have even designed a small PCB for this, so building it shouldn’t be a problem at all. This certainly isn’t an original circuit, and is here mainly as an addition to the ‘Dimmer with MOSFET’ article elsewhere in this website. The design has therefore been tailored to this use. A frequency of 500 Hz was chosen, splitting each half-period of the dimmer into five (a low frequency generates less interference).

Picture of the project:

The first timer is configured as a standard astable frequency generator. There is no need to explain its operation here, since this can easily be found on the Internet in the datasheet and application notes. All we need to mention is that the frequency equals 1.49 / ((R1+2R2) × C1) [Hz] R2 has been kept small so that the frequency can be varied easily by adjusting the values of R1 and/or C1. The second timer works as a monostable multivibrator and is triggered by the differentiator constructed using R3 and C3.

Parts layout:

The trigger input reacts to a rising edge. A low level at the trigger input forces the output of the timer low. R3 and C3 have therefore been added, to make the control range as large as possible. The pulse-width of the monostable timer is given by 1.1xR4xC4 and in this case equals just over a millisecond. This is roughly half the period of IC1a. The pulse-width is varied using P1 to change the voltage on the CNTR input. This changes the voltage to the internal comparators of the timer and hence varies the time required to charge up C4.

Circuit diagram:

he control range is also affected by the supply voltage; hence we’ve chosen 15V for this. The voltage range of P1 is limited by R6, R7 and R5. In this design the control voltage varies between 3.32 V and 12.55 V (the supply voltage of the prototype was 14.8 V). Only when the voltage reaches 3.51 V does the output become active, with a duty-cycle of 13.5 %. The advantage of this initial ‘quiet’ range is that the lamp will be off. R8 protects the output against short circuits. With the opto-coupler of the dimmer as load, the maximum current consumption of the circuit is about 30 mA.

Power supply:
COMPONENTS LIST
Resistors:
R1 = 270k
R2,R3 = 10k
R4 = 100k
R5,R8 = 1k
R6,R7 = 220R
P1 = 2k2, linear, mono
Capacitors:
C1,C4 = 10nF
C2,C5,C6 = 100nF
C3 = 1nF
C7 = 2µF2 63V radial
C8 = 100µF 25V radial
Semiconductors:
D1 = 1N4002
IC1 = NE556
IC2 = 78L15
Miscellaneous:
P1 = 3-way pinheader
K1 = 2-way pinheader

Author: Ton Giesberts Copyright: Elektor Electronics

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Discrete Robot

This simple robot, which responds to light and avoids obstacles, can be built without using a microcontroller, programmer or PC. The only ‘special’ component in the circuit is a window discriminator (a fancy version of a window comparator). Resistors R1 and R2 in combination with light-dependent resistors LDR1 and LDR2 form a voltage divider (with the current being limited by R1 and R2 for bright light). Window discriminator TCA965 compares the mid-point voltage with an upper threshold value (adjustable using P1) and a lower threshold value (adjustable using P2). Outputs AU, AI, AO, and AA go High if the voltage lies below, inside, above or outside this window, respectively; otherwise they remain Low.

Output AA switches transistor T1, which drives the right-hand motor. The light-dependent resistors can be attached on the left and right sides of the vehicle, or at the front and rear. This causes the robot to turn to the right, due to the motor on one side being stopped, until the desired lighting relationship is restored. The vehicle will then continue to travel in a straight line until the lighting relationship again changes, at which point it will again turn, and so on. You can experiment with various behaviour patterns by using the other outputs of the window discriminator. If a transistor is provided for each of the AU and AO outputs of the TCA965, the robot can be made to travel toward or away from a light source, depending on the connections.

Using the window discriminator, the robot will operate under the rules of a three-point controller (left, straight ahead, or right). If you fit the light-dependent resistors in a box under the vehicle together with a light source, you can try to have the robot follow a black line on a white background. A reflective IR sensor enables the robot to respond to obstacles. This not as simple as it might seem, since the Sharp IS471 operates the IR LED with pulsed light and uses sophisticated detection processing. When an obstacle is detected, the output (pin 2) goes Low and blocks transistor T2. This causes the motor to stop, and the vehicle will rotate about the stationary wheel until the obstacle is no longer in its path.

Circuit diagram:

Discrete Robot Circuit Diagram

The sensitivity of the IS471 can be set using P3. As its range is only around 10–15 cm, the vehicle must not travel too quickly, since otherwise it will not be able to avoid obstacles in time. This part of the circuit is also open for experimentation. If a relatively large and fast robot requires an obstacle detector (or isn’t fitted with the IS471), an ultrasonic detector can also be used. Suitable complete construction kits are available from Conrad, for example. You can also fit a suitable mechanical pushbutton switch mounted on a flexible rod. The obstacle detector can also drive a warning buzzer or a lamp; the circuit leaves lots of room for your own ideas. The circuit works over a wide range of supply voltages from 4.5 to 16 V.

If larger motors are used, transistors with increased power-handling capacity and heavier batteries are necessary. The author connected two 4.8-V rechargeable batteries in series and used BC388 transistors as drivers for Lego micromotors. You can build the robot entirely according to what you have in your parts box. The mechanical elements can also be freely selected, but they partially determine the behaviour and operation of the robot. The author’s robot is made from a Lego chassis with a prototyping board holding the circuitry attached using elastic bands. The motors are fitted on the left-hand and right-hand sides. The third wheel at the front can turn freely.

One problem must be mentioned: if an obstacle is detected while an incorrect lighting relationship is present, the vehicle remains standing. In this case, a bit of logic could be added to cause both motors to rotate in reverse. However, that would require directional switches for the motors or motor driver ICs (L293D). The simple circuit would become more complicated and larger, and at some point you would end up using a microcontroller after all - but that’s just the point of the story.

Author: Gerhard Nöcker - Copyright: Elektor Electronics

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Relative Field Strength Meter For A DMM

Many passive field strength meters have appeared in the past, typically using a 50mA analog meter movement if reasonable sensitivity was to be obtained. This circuit is similar but has the advantage that it works with the high-impedance load of a digital multimeter, typically switched to the 200mV range. The sensitivity is adequate for low power equipment like CB radios, cordless phones and model R/C sets (cars, model airplanes, etc).

For best results, use OA81 or similar germanium diodes. Modern Schottky signal diodes could also be used but the results are not as good. The circuit can be wired directly into a small plastic box with protruding banana posts to match the terminals on your DMM. A banana jack can also be used for the antenna which could be just a 500mm length of wire as a starting point.

Circuit diagram:

 Relative Field Strength Meter Circuit Diagram For A DMM

 

Author: Gerard La Rooy Copyright: Silicon Chip Electronics

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Simple Combination Lock Circuit

This simple combination lock accommodates codes from 1-9 digits long, with the only restriction being that the same digit cannot be used twice. The circuit shows strapping for a 4-digit code, in this case "2057". Any unused switches are strapped to ground. When power is applied, the 330nF capacitor connected to pin 1 of inverter IC1a is discharged, holding it at a logic low level. The high output is inverted by a second gate (IC1b), with the result being a logic low on pin 4. This pulls Q1’s emitter low via D1, causing the transistor to conduct. The falling voltage on the collector then pulls the input of IC1c low, which in turn resets counter IC2.

On reset, output O0 (pin 3) of IC2 goes high, charging the 330nF capacitor via D2 and the 33kΩ resistor. If switch S2 is now pressed, Q2's emitter will be pulled high and so Q2 conducts, applying a rising positive voltage to one end of the 1MΩ resistor. This resistor and the 33nF capacitor act as a switch "debounce" circuit, delaying the pulse through IC1e by about 33ms. After the delay, the output of IC1e goes low. However, counter IC2 does not increment at this stage, since it needs a positive-going edge at the clock input (pin 14). When the switch is released, Q2 turns off, IC1e’s output goes high after the debounce period and the counter advances to the next state (ie. O0 goes low and O1 goes high).

Circuit diagram:

Simple Combination Lock Circuit Diagram

When output O0 (pin 3) goes low, the 330nF capacitor starts discharging through the 33kΩ and 10MΩ resistors. This allows about 3s for the operator to press the next button. If no button is pressed within this period, IC1b’s output goes low, which pulls Q1’s emitter low and resets the counter via IC1c. Hence the code entry must be restarted. When the second digit of the code is entered (0 in this example), Q2’s emitter is again pulled high. Q2 thus turns on and after the debounce delay, IC1e’s output goes low. When the switch is released, Q2 turns off, IC1e’s output goes high and the counter advances to state 2.

Note that while the switch is pressed, IC1d’s output is high, recharging the 330nF capacitor and therefore resetting the 3s delay. Thus, the operator is allowed another 3s to press the next digit. This process is repeated for each digit in the sequence. If the wrong switch is pressed at any point, IC2 is reset as described above. Conversely, if the correct code is entered, IC1 advances to state 4 (for our 4-digit example) on release of the fourth switch. Output O4 then goes high and turns on Q3 and relay 1. Q3 can handle up to about 300mA of load current. If more current is required, then either a Darlington or power Mosfet can be substituted. D4 is required if the load is inductive (eg, a relay, solenoid, etc).

Author: Len Cox - Copyright: Silicon Chip Electronics

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Zero Gain Mod For Non-Inverting Opamp

Electronics textbooks will tell you that a non-inverting opamp normally cannot be regulated down to 0 dB gain. If zero output is needed then it is usual to employ an inverting amplifier and a buffer amp in front of it, the buffer acting as an impedance step-up device.

The circuit shown here is a trick to make a non-inverting amplifier go down all the way to zero output. The secret is a linear-law stereo potentiometer connected such that when the spindle is turned clockwise the resistance in P1a increases (gain goes up), while the wiper of P1b moves towards the opamp output (more signal). When the wiper is turned anti-clockwise, the resistance of P1a drops, lowering the gain, while P1b also supplies a smaller signal to the load. In this way, the output signal can be made to go down to zero.

Circuit diagram:

Zero Gain Mod Circuit Diagram For Non-Inverting Opamp

Source : www.extremecircuits.net

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Reset Sequencer Circuit

It is often necessary in complex designs to provide a sequence of reset pulses to different parts of a circuit to ensure the whole design functions reliably. The DS1830 from Maxim (www.maxim-ic.com) provides three sequenced open-drain reset outputs. This chip is designed for 5 V systems but a 3.3 V version (DS1830A) is also available. Both are offered in a range of package outlines including DIP, SO and µSOP. Two inputs give the chip some degree of programmability of its characteristics: The TOL input defines the chips tolerance to power supply fluctuations before a reset sequence is triggered.

Circuit diagram:

Reset Sequencer Circuit Diagram

TOL	5V	3,3 V	TD	Tr1	Tr2	Tr3
+ Ub	Ub .0.95	Ub .0.95	0 V	10 ms	50 ms	100 ms
0 V	Ub .0.90	Ub .0.90	Open	20 ms	100 ms	200 ms
Open	Ub .0.85	Ub .0.80	+ Ub	50 ms	250 ms	500 ms

Jumper JP1 allows the TOL to be connected to Ub (Vcc), ground or left open circuit and will result in the following three reset thresholds: The TD input allows the length of the reset signal to be programmed and jumper JP2 gives the following three possibilities: The PBRST (pushbutton reset) allows a manual reset button to be connected to the chip. This input has a built-in 40 k? pull up resistor and can also be driven by a digital output or used to cascade additional devices to provide more sequenced reset signals.

Author: Gregor Kleine Copyright: Elektor Electronics

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Opamp With Hysteresis

At first glance, the circuit in the diagram does not look out of the ordinary, and yet, it is. This is because it combines two characteristics that are usually assumed to be incompatible: hysteresis and a high input impedance. In a standard op amp circuit, this is, indeed, true, because the creation of hysteresis is normally achieved by positive feedback to the +ve input of the op amp. Unfortunately, the requisite resistance network causes a drastic deterioration of the original high input impedance of the op amp. So, when a high input impedance and hysteresis are wanted, the solution is to obtain the needed positive feedback by coupling the resistor network not to the +ve input but to the offset correction pin.

Circuit diagram:

Operational amplifier With Hysteresis Circuit Diagram

When this done, the hysteresis so obtained is calculated from Uh = 1.2/R4Uo, where Uh is the hysteresis voltage and Uo is the output voltage of the op amp, both in volts The value of R4 must be in kΩ. The level of Uo depends, of course, on the load.

Source : www.extremecircuits.net

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Dual Relay Driver Board Circuit Schematic

A simple and convenient way to interface 2 relays for switching application in your project. This relay driver boosts the input impedance with a regular BC547 NPN transistor (or equivalent). Very common driver. It can drive a variety of relays, including a reed-relay.

Transistor Q1and Q2 are a simple common-emitter amplifier that increases the effective sensitivity of the 12 volt relay coil about a 100 times, or in other words, the current gain for this circuit is 100. Using this setup reduces the relay sensitivity to a few volts. R3 and R4 restricts the input current to Q1 and Q2 to a safe limit. Diodes D3 and D4 are EMF dampers and filter off any sparking when the relay
de-energizes.

Picture of the project:

 Front View Of Dual Channel Relay Board Driver

Circuit diagram:

Parts:

R1-R2 = 1K
R3-R4 = 5.6K
C1-C2 = 100nF-63V
D1-D2 = Red LED
D3-D4 = 1N4001
L1-L2 = 12V Relay
Q1-Q2 = BC547

Specification:

• Input - 12 VDC @ 84 mA
• Output - two SPDT relay
• Relay specification - 5 A @ 230 VAC
• Trigger level-2~5VDC
• Berg pins for connecting power and trigger voltage
• LED on each channel indicates relay status

Source : www.extremecircuits.net

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Logic Probe With Sound Circuit

This logic probe can be selected to operate on TTL or CMOS logic levels, depending on switch S1. A string of resistors associated with switch S1 sets the threshold levels for a window comparator comprising IC1a and IC1b. Depending on whether the level applied to the probe is high or low, the window comparator turns on LED1 (high) or LED2 (low). The 1.2M and 680k resistors set the probe signal to a midrange value when the probe is open-circuit, thereby preventing either LED from being lit.

Circuit diagram:

Logic Probe With Sound Circuit digram

If a pulse signal is present, the output of IC1a will toggle the clock input of flipflop IC2a. This drives LED3 which either lights for each pulse or continuously, depending on the setting of switch S2. Finally, the outputs of IC1a & IC1b are connected by diodes D5 & D6 to the base of transistor Q1 which is connected to the Reset input of flipflop IC2b. This has a piezo sounder (not buzzer) connected between its Q and Q-bar outputs so that it produces a sound which echoes the input pulse signal.

Author: Tom Hughes Copyright: Silicon Chip Electronics

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