A more common way to solve this would be to connect the potmeter with R5 to the non-inverting input and the signal to the inverting input. As light levels drop R2 increases resistance with Vref falling below Vin turning on a relay though an opto-coupler to turn on lights or whatever. This matters for the hysteresis. Use the typical comparator circuit to compare two voltages. Hi, I am working on using lm as a battery charger, I need a circuit that can detect when the battery is either low or charged and switch on a relay, can you help with a good circuit? It is a good idea to use a flyback diode across the coil of your relay to protect your transistor and other components, like what has been suggested.

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KyranF 5, 8 Until that moment the output is high, so that.

LM339 Quad Voltage Comparator IC Design Kit (#1405)

You could try to use the voltage level detector circuit see the infrared reflector detector circuit using LM picturewhere you could replace the infrared with the green LED come from another circuit. By setting a trip point with the 10K pot one can make an under-voltage indicator. I dont need a fancy answer-just an answer that show how to use LMN in pm339 clear way to make the circuit.

The simplest circuit would be to connect it as an emitter follower, with the LED and series resistor on the emitter positive side. The maximum current that could flow on this Q8 transistor output sink current according to the LM datasheet is about 18 mA.


A snubber is usually an RC circuit used to suppress the rise of voltage or current to protect contacts of a relay from welding. Sign up using Email and Password. The equation is derived from an exponential RC time delay, you could read more information of how to derive rekay equation in National Semiconductor Application Note So when the main computer tells the fuel pump computer that it is time to raise the pump lm3339 up to full 12V operation, it sends out this 5V signal.

Sign up using Email and Password. The series resistor is not required. As light levels drop R2 increases resistance with Vref falling below Vin turning on a relay though an opto-coupler to turn on lights or whatever.

This would also give the comparator output more swing, yielding more hysteresis. Why did you say “this matters for the hysteresis”? Thanks Again, Stu rezer said: A silicon PN junction has about a 0. Also–“Audioguru”can you recommend a different Comparator that may better handle this current?

This will require the output to be 1.

Comparator Circuits Examples Tutorial

Chetan Bhargava 4, 5 21 I got tired on reading every single one. Some example of this situation is to send the interrupt signal to the microcontroller operation when the temperature is already exceeds certain limit or the light intensity exceeds certain bright level.

You may want to put a diode in series with relat transistor base as well since the output of the LM will not saturate to 0V.


As the resistance of the thermistor goes down, it increases the voltage of the comparator’s positive input pin 3. If we adjust the 10K pot to where Vin is greater than Vref the output on pin 7 goes to about plus volts forward biasing the 1N and turning on the LED.

You could read more about LFR on this following articles:. Its feedback and input resistors are internal. The comparator is powered by a 9v 1a supply, and there is an LED in parallel with the relay, thats how I noticed the half on – the LED would be dimly lit while the relay was switching. I’ve updated the question to indicate the type of thermistor and actual temperatures and resistances.

The diagram in picture no. And these relays can be had with spike suppression diodes built in. Pete The threshold is adjusted with the resistor divider – you can simply use a potentiometer wiper with 5v to the top of pot, gnd on the bottom as a variable V reference.

There’s no reason that a single 2N shouldn’t be able to sink enough current to turn on your relay. Aug 16, 5. This will impact the reference divider and hysteresis component value selections.