LM3914 12V Battery Monitor Circuit

This bar graph LED battery level indicator circuit is based on LM3914 monolithic IC from National Semiconductor that senses the voltage levels of the battery and drives the 10 light emitting diodes based on the voltage level that is detected.

To calibrate the circuit it must be connected to an adjustable regulated power supply.
Connect an input voltage of 15 volt between the positive and negative poles and adjust the 10K preset until Led 10 lights up. Lower the voltage and in sequence all other Led’s will light up. Check that Led 1 lights up at approximately 10 volts.

This circuit to your own needs by making small modifications. The circuits above is set for ‘DOT’ mode, meaning only one Led at a time will be lit. If you wish to use the ‘BAR’ mode, then connect pin 9 to the positive supply rail, but obviously with increased current consumption.

The LED brightness can be adjusted up- or down by choosing a different value for the 3K9 resistor connected at pin 6 and 7.

You can also change the to monitoring voltage level.

For example, let’s say you wanted to change to 12 – 15 volt,
Remove the R2 resistor and connect 15volt to the input (+ and -) and adjust the 10K potentiometer until Led 10 lights up. Connect  200 Kilo-ohm potentiometer at pin 4 and -. Reconnect a voltage from 12 Volt to the input. Now adjust the 200K potentiometer until Led 1 lights up. When you are satisfied with the adjustment, feel free to exchange the 200K potentiometer with resistors again.(after measuring the resistance from the pot, obviously).

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Analog Frequency Meter Circuit using NE555

This 1-kHz linear-scale analog frequency meter circuit uses the 555 as a pulse counter.

2N3904 NPN General Purpose Amplifier

VR1 Adjust for 1kHz full scale reading.

Frequency is read on M1, (or 1mA meter) which can be calibrated to read 0 to 1 kHz.

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Sensitive RF Voltmeter Probe Circuit

This Circuit measures RF voltages beyond 200MHz and up to about 5V.

The Diode should be mounted in a remote probe, close to the probe tip. Sensitivity is excellent and voltage less than 1V peak can be easily measured. The unit can be calibrated by connecting the input to a known level of RF voltage, such as a calibrated signal generator, and setting the calibrate control.

VR1 =For  Calibrate

VR2 = For Zero set

MPF102 N-Channel JFET VHF Amplifier Pin Configuration

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Capacitance Meter Circuit Using 555

It directly reads capacitance in the range 100pF to 10µF. IC1 and IC2 operate as an astable (with frequency above 80Hz) and as a monostable multivibrator respectively. Time period of the monostable multivibrator is determined by the resistors (R3 to R7 and VR1 to VR5) selected by switch S1 and unknown capacitance Cx.

By using high accuracy multi-turn presets the capacitance meter can be calibrated for the different ranges. Adjust VR6 so that the meter gives full scale deflection. Now the meter is ready for the calibration.

Cx should be 3.3µF, 0.33µF, 0.033µF, 3300pF, 330pF in ranges 1, 2, 3, 4 and 5 respectively for calibration. In each range the corresponding preset is adjusted to read 3.3 on the meter. Once presets have been set they should not be disturbed.

Electrolytic capacitor should not be used because of their wide tolerances. Supply should be well regulated.

Unknown capacitor is placed in the position of Cx and S1 is rotated to get a clear reading. It is not possible to go above 10µF as value of presets goes below IC555 specifications.

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Digital Probe is an essential instrument for any electronic engineer working on digital circuits.The discribed here can show all the four digital states through two LEDs.It can be used for both TTL and CMOS ICs.

The probe has two LEDs red and green. When the probe is not attached to anything, or if it is thuched to a point which is in tri-state, the green LED will glow dimly. The green LED will stop glowing if the probe is on ‘low’ logic state. If the probe is put on a ‘high’ logic state the green LED will glow brightly. The red LED will glow when a pulse train is present. However, if a very slow pulse is present then the green LED will go on and off but the red LED may not glow.

The probe will take power supply from the device under test. As the power requirement is about 7 mA it should not pose any major problem to the power supply of the device under test.

A forward biasing resistor (R1) of high resistance is connected to the case of transistor T1. Due to this resistor a small current passes through the resistor-diode limb (R2,D2) and the green LED glows dimly.

When the probe touches a ‘high’ logic point, T1’s collector current increases and the LED starts glowing brightly. Presence of a low logic state stops flow of current through T1’s collector circuit and the LED stops glowing.

When a pulse is present emitter of T1 goes alternately ‘high’ and ‘low’. The resulting AC component of the pulse is passed through capacitor C1. this current gets rectified by D3 and red LED glows. Input impedance of the circuit is of the order of 100k.

As the circuit uses very few components, it can be constructed on a general-purpose PCB. For ease of use, LEDs should be placed near the probe. Power supply cables should be placed at the farther end from the probe.


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]]> http://www.electronicecircuits.com/electronic-circuits/ultra-simple-digital-probe/feed 1 http://www.electronicecircuits.com/electronic-circuits/simple-battery-state-indicator http://www.electronicecircuits.com/electronic-circuits/simple-battery-state-indicator#comments Thu, 17 Sep 2009 14:58:16 +0000 admin http://www.electronicecircuits.com/?p=432

Many a time the outdoor audio or video recording becomes imperfect due to a ‘dying’ battery. If the battery voltage is less than 9V for a 12V recorder, the output during playback will not be of a good quality due to variations in the motor speed.

Many car owners take their battery for granted, rarely giving it the maintenance required. As the winter nights advance the demands made on this vital power source increase. Combined with the inevitable aging process and diminishing ability to store a charge for a  long period, this makes the requirement of a simple aid to monitor battery voltage continuously obvious.

The indicator was therefor designed to forestall any incipient failure by providing ‘at a glance’ information on battery state with three coloured LEDs.
Green indicating a battery voltage adequate for normal use,
Yellow that the voltage was fairly low and
red that the cell was dead.

When the battery is in top condition, its output voltage will be around 13V and, of course, even higher if recently charged. This potential is applied via D8 through R4, D7 and R5 to the base of Q2. This turns Q2 on, causing D6 to illuminate via R3. At this time Q2 effectively places a short circuit across the rest of circuit via D6, preventing D2 and D3 from emitting light.

As the battery voltage becomes lower,Q2 begins to turn off as the threshold of D7 is reached.This allows D2 to conduct as Q1 has all the time to be turned on via R1, D1 and R2. (D5 increases the voltage required for turning on D2 when T2 is on.) The current thus drawn via R3 precludes D3 from illuminating as the potential across it and D4 is not above the zener level.

Eventually, at still lower battery voltage, Q1 will turn off in the same way as Q2, allowing potential at the junction of D2, D3 and D6 to rise in excess of 5V zener level of D4 which begins to pass current and illuminates D3.

Zener diodes may be selected for other switching points and / or battery voltages.


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The microampere meter

The microampere meter gives full scale deflection for 0.1 V input. The current to be measured is passed through a known resistance R and the voltage drop across it is measured.

The this table shows the relationship between different values of R5 and the current that will give full scale deflection.

A high impedance Micro Ampere meter using a 741 .The operational amplifier is used as a non-inverting dc amplifier in which the negative feedback is through a dc meter requiring 1mA for full scale deflection.

Diodes D1 and D2 protect the IC from accidental excessive input voltages and diodes D3 and D4 protect the meter from overloads.

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Zener Diode Tester Circuit

This tester helps to check the voltages of zener diodes. It is very inexpensive and  handy to find the voltages of small glass zeners, whose markings get rubbed-off very easily. The 741 op-amp has been used in differential mode.

The 47k linear potentiometer’s dial is calibrated in terms of voltages between 0 and 27V, either by using various known zeners in the range, or by applying a known voltage at pin 3 of IC 741. The position of the potentiometer where the LED gets turned off, gives the breakdown voltage of zener.

The power supply voltage can be between 9V and 30V. But zener to be tested, and should not be changed once the dial is calibrated.

You can also check the polarity of zener. The cathode side should be connected to the red clip.(i.e. pin 3 of IC 741) If you connect the zener the other way round, then it will just behave as a diode, and the LED will remain off at all positions of the potentiometer.

In case of a zener that is shorted from inside, the LED will remain off with zener connected either way ti the clips. In case of a zener that is open-circuit from inside, the LED keeps glowing at all positions, with zener connected either way to the clips.

You may even differentiate between a diode and a zener by their behaviour on this versatile tester.

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Low resistance measuring meter circuit

The circuit for a LRMM described here is simple and has the following advantages over other meters:

* No zero knob (see it once and forget it forever).
* Scale reading is from zero to a fixed value rather than infinity.
* LRMM uses a 1.5-volt penlight cell, two scales (0-1 ohms and 0-10 ohms) over a dial and a push-to-on switch due to large power consumption by the circuit.

A constant current generator T1 passes a known current through the resistance to be measured. A maximum drop of 100 mV across the emitter of T1 and ground is displayed on the meter whose internal resistance is much higher than the testing resistance (maximum 10 ohms), due to which the meter does not load the circuit. The diode across the absence of a testing resistance.

The bias for the transistor is provided by R1, VR1, R2, R3, D1, and R4. Using diodes D1 and D2 helps in holding the bias level constant inspite of decaying battery.

The meter should have a 0-500µA linear scale. Any general-purpose meter can be used with a shunt resistance. T1 can be any silicon npn transistor with a high current gain factor.

Adjust the instrument by shorting probes A and B. Otherwise it shows a zero resistance. Adjust in 0-10 ohms scale first. Other adjustments follow automatically. This circuit can be easily built within an hour.

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