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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Loudspeaker, Speaker Audio Power Indicator For Power Amplifier

The LM3915 audio power meter with a bar-type read-out. The circuit has one drawback in that it requires a separate power supply. This is compensated, however, by the fact that it is pretty sensitive (0.2W min.) and does not degrade the sound quality in any way, since it does not present an additional load to the amplifier (in contrast, many inexpensive AF power indicators derive their display current from the amplifier).

The value of resistor R1 depends on the loud-speaker impedance as shown in the table inset in the circuit diagram. The resistor may be replaced by a wire link in the relevant position on the PCB if it can be fitted inside the plug that connects the indicator to the loudspeaker. This makes it convenient to use the indicator with loudspeakers of different impedance: use a dedicated cable for each impedance.

For use with stereo systems, the circuit is either built in duplicate or the signals across the loudspeakers are applied to two series-connected resistor R1, whose common junction is connected to pin 5 of IC1. The latter method may raise some eyebrows, but it works fine in practice.

The power supply for the indicator is derived from a simple a.c. mains adaptor that provides a d.c. output of 12-20V.

Finally, it should be noted that the actual power measurement is an approximation only since the LM3915 reacts to the positive half-cycles of the signal only. This causes the top LED in the bar to light at a slightly reduced intensity.

LM3915 IC DATA

Notes:

• Capacitor C1 is required if leads to the LED supply are 6″ or longer.
• Circuit as shown is wired for dot mode. For bar mode, connect pin 9 to pin 3. V_LED must be kept below 7V or dropping resistor should be used to limit IC power dissipation.
• V_REF=1.25V{1+(R2/R1)}+R2×80µA
• I_LED=(12.5V/R1)+(V_REF/2.2kΩ)

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Crystal Tester

We have crystals lying about, but don’t know whether these are still working all right. The crystal tester described here will quickly show whether a crystal can be used or should be discarded.

Transistor Q1 and the crystal under test from an oscillator. Capacitors C1 and C2 form a voltage divider in the oscillator circuit. If the crystal is in good order, the oscillator will work. Its output voltage is then rectified and smoothed by D1 and C4 respectively. The resulting direct voltage at the base of Q2 is sufficient to switch this transistor  on, so that the LED lights.

The circuit is suitable for use with crystals of a frequency between 100kHz and 30MHz. Current consumption is about 50mA.

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Useful to monitor audio power delivered to loudspeakers
No power supply – no setup required

This device, connected to the loudspeaker output of an audio amplifier, will indicate the instantaneous output power delivered to the loudspeaker(s) by means of six LEDs illuminating one after another by voltage values increasing little by little, providing the visual impression of a luminous bar or column, increasing and decreasing in height following the increase and decrease of the signal’s level.

The input signal is first rectified by D1 and then sent to six different voltage dividers, one for each LED. In this way, the indication provided by the LEDs illumination of this “Power Display”, will be related to the instantaneous power sunk by the whole loudspeaker cabinet. Six output power levels are displayed by the LEDs in a 2W – 80W range (no setup required). Each nominal power level indication into 8 Ohms load is reached when the respective LED illuminates at full brightness.

Notes:

• The output power indicated by each LED must be doubled when 4 Ohms loads are driven.

• The circuit can be adapted to suit less powerful amplifiers by reducing the number of LEDs and related voltage dividers.

• LEDs of any dimension can be used, but rectangular shaped devices will be more suitable to be compacted in bars or columns.

• For a stereo amplifier, two identical circuits are required.

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]]> http://www.electronicecircuits.com/electronic-circuits/six-led-bar-power-indicator/feed 0 http://www.electronicecircuits.com/electronic-circuits/ultra-simple-digital-probe http://www.electronicecircuits.com/electronic-circuits/ultra-simple-digital-probe#comments Sun, 20 Sep 2009 01:46:48 +0000 admin http://www.electronicecircuits.com/?p=462

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/auto-heat-limiter-for-soldering-iron http://www.electronicecircuits.com/electronic-circuits/auto-heat-limiter-for-soldering-iron#comments Sat, 19 Sep 2009 07:39:43 +0000 admin http://www.electronicecircuits.com/?p=451

Usually a soldering iron takes a couple of minutes to get adequately heated up to melt the solder,after which the heat generated is much above the requirement and is wasted. Moreover, excessive heat decreases the life of the bit and the element, causing serious damage to the delicate ICs, capacitors and PCB tracks.

A common solution to this problem is to use a diode in series with the mains and the load, so that the iron gets only half the AC cycle. But this process has one serious drawback, that it slows down the normal rate of heating. Thus, after switching  on the power, one has to wait for a long time to start work. This can create a lot of annoyance especially when one is in a hurry.

The circuit described here solves this problem in a simple and inexpensive way and can be used with various types of loads of up to 80 watts. As the mains is switched on, an approximate 15V drop of the positive half cycle across R5 is detected and supplied to T1, which acts as a voltage regulator.

Zener diode D6 together with diode D4 (yellow LED) stabilises the emitter voltage of T1 at 13.2V DC which is then delivered to the relay circuit built around T2 and C3. Capacitors C3 charges through the base-emitter path of T2 and causes the relay to actuate. which in turn allows both the half cycles of the AC mains to flow through diode D3 and R5 to the load to heat it up at a normal rate.

After a certain lapse of time (about 2 minutes preset ), C3 saturates and T2 stops conducting through the relay, thus switching on series diode D2 to allow only half of the AC cycle through the load.

After switching off the system, C3 discharges very slowly through R3 and R4. Before C3 gets completely discharged, if the power is switched on again, C3 takes a shorter time to reach the saturation level, thus switching series diode D2 much earlier than the preset time to prevent double heating of the load.

However, if the circuit is switched on only after a few seconds of switching off, C3 gets no time to discharge and the relay does not actuate at all. MOreover, if the relay circuit fails due any reason and T2 does not conduct, no harm is done to the load because in that case D2 remains in series with it. Thus the circuit offers complete protection to the load.

As stated earlier, the given value of C3 gives a delay of 2 minutes. However, a 1000µF capacitor can also be used to produce a 41/2 minutes delay. R5 maintains a drop of about 15V across itself. So, for use in different load conditions, its value changes as shown in table 1.

The whole circuit can be mounted on a PCB and fitted in an adapter case (7.6cm *5.1cm * 6.4cm) and used as a mains plug. Since R5 gets heated up during the operation, it should be kept well isolated from other components.

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]]> http://www.electronicecircuits.com/electronic-circuits/auto-heat-limiter-for-soldering-iron/feed 4 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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