Voltage Controlled Switch


Introduction: Voltage Controlled Switch

 This is a little circuit that i designed that could be used for a charging controller or voltage limiter. Its very useful if you are thinking of making a solar charger. 

Step 1: Get the Stuff

To build this circuit you would need the following:
2 1k ohm resistors 
2 5V zener diodes
A comparator (the one it used is a KA393)
A led
A Project Board / Breadboard
A Digital Multimeter
A variable power supply ( of your choice once its in the limits of the comparator)
Some wire

Step 2: Assemble the Circiut

 In the assembly of the circuit u can alter the circuit as you please. For instance i didn't go as exact as the schematic.
Just follow the diagram below.
Just in case you don't know how the comparator works heres a link that shows how.

Step 3: What Its Suppose to Do

 In the circuit that i build thats not totally the same as the in the diagram, is suppose to turn on the led at 8V because i used a 5v and 3v zener. I also used a two 9 V batteries which voltages differ. The circuit i suppose to turn the led on at 8 V and any voltage lower than it should go off. 

Step 4: Tips

 If turn the comparator positive and negative leads around will have the reverse effect. I mean instead of  turning on at 8V it will turn off and any voltage thats lower than that it will be on. Sorry i don't have a picture of that particular instance, but i will put in one any way. [ for decorative purposes]
Also to increase the current of a load which it quite large the LED can be replaced with a P-MOSFET which will give you the gain.

Step 5: Thanks

 Thanks for looking at this and i hope that it was helpful .Have a good and GODLY day.



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    38 Discussions

    I am making a 100v dc how do you make a comparator work with that voltage? I thought making a voltage divider then go off that. Any ideas?

    1 reply

    Yes u need a 2 voltage dividers and u can still use a zener diode tapped on the dividers as the reference voltage, the other voltage divider you can precalculate the resistor values using the voltage divider formula or use a potentiometer to vary and test the circuit, to make the comparator work u need to use a voltage regulator chip like the LM340T, sorry when off track above , use a suitable resistor in front the IC to reduce the current, enough as to not kill it, or u could use a buck converter board to reduce the voltage to the ic

    I think what ur asking is if the voltage is below 8 volts u want 1 led to light up and above that another led lights up? The comparator switches between high and low, which is that in one state the output is giving u a negative polarity n in the other state positive polarity, so the led is a diode and it only lights when it gets the right polarity between anode n cathode, so at the output place a resistor suitable for the current the led needs, if bigger than 25ma current place a transistor there, so say u have 2 leds green n red take the green led place the cathode end to the negitive or ground side of the circuit and the anode to the output resistor, like wise with the red led place the anode side to the positive end of the circuit n cathode side to the output resisto, if this hasnt answered your question pls be more specifc in detail of what ir trying to do

    I did not understand which component you used and why the 5V supply? Can't we use a comparator that derives the required power from the charge capacitor? Will you please explain so that it'll be easy for me to make this circuit. Please help..

    this a prototype circuit and you see how complicated it is right now, there isnt even all the components need to make it work yet.

    prototype circuit.JPG

    Please help me in designing the circuit...

    Thank u for the reply. The problem is that the solar panel I am using only give 1.5 V output. So I think may be I need to use some sort of amplifier because I can not use any external power supply. And why do we need bi-stable multivibrator? Please help me by drawing up a schematic that works for me. It would be a great help...


    I need to make a circuit working between 1 V and 2.4 V. I want to make a solar charger that charges a capacitor until it reaches 2.4 V, once the voltage reach 2.4 V, the switch is closed and the led lits up. At the time, 1st capacitor is discharging, I want to charge another capacitor. When the 1st capacitor discharges to 1 V, then I want to charge the 1st capacitor again and connect the load to 2nd capacitor. Please help me with this..I'll be very thankful to you.

    2 replies

    O sorry i didnt read it so properly you also need a Bi- stable multivibrator in the circuit for it work

    the problem i see with this is that most opamps need @ least 3v to work properly. so you may have to run an external power supply or a larger voltage solar panel to power the opamp the rest is easy. i draw up a schematic for you.

    THanks for posting this, right along the lines of what I am looking for. What would I need to change to be able to control a realy with this circuit. Specifically, have a SPST relay engange at 5V or higher, or relay disengage at anything under 5 volts? Any help is appreciated.

    2 replies

    O yea the K represents 1000. So 1k is 1000 ohms and 10K is 10000 ohms. ZD is the zener diode

    hi crinton, sorry i didnt update this thing yet but there is an easier way of doing it. You can use one diode and a voltage divider plus an opamp / comparator. Ive placed a schematic on below it will show you how to connect it. The circuit is currently connected to turn on @ 5V, if you want it to do the opposite turn off @ 5V exchange the positions of pin 2 and 3. i hope you know how to use a transistor. I labeled ther transistor points B for base, C for collector and E for Emitter. Just purchase any npn low powered npn transistor and you will be fine. Transistors come in 3 different pin configurations CBE, BCE, ECB. the package will show you the pin out. if not go and reference it online. if i missed any thing message me back.

    comparator circuit.bmp

    this is a very nice I thanks i dont understand it all but im going to collage and will ask some friends to help me out wonderful place for me to start though thanks.

    how about if i want to use it in solar charger for one AA battery? please help me, huhu..