Introduction: Voltage Controlled Switch

Picture of 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

Picture of 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

Picture of 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

Picture of 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

Picture of 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.


ArturoC6 (author)2016-07-21

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?

josh1001 (author)ArturoC62016-07-21

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

itsandyhorton (author)2015-02-17

How would you make it light up a different LED if the Voltage is below 8?

josh1001 (author)itsandyhorton2015-02-21

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

Elexa (author)2013-01-30

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..

josh1001 (author)2013-01-28

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.

Elexa (author)2013-01-27

Please help me in designing the circuit...

Elexa (author)2013-01-24

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...

Elexa (author)2013-01-23


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.

josh1001 (author)Elexa2013-01-24

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

josh1001 (author)Elexa2013-01-24

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.

Crinton (author)2012-04-23

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.

josh1001 (author)Crinton2012-04-28

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

josh1001 (author)Crinton2012-04-28

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.

srainsdon (author)2010-03-19

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.

awatif1989 (author)2010-03-07

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

josh1001 (author)2010-02-10

 when u open the picture view it with the windows picture and fax viewer

rpvanpatt (author)josh10012010-02-10

 hey, sorry i dont have that program

josh1001 (author)rpvanpatt2010-02-11


josh1001 (author)josh10012010-02-11

 can u see this one?

rpvanpatt (author)josh10012010-02-15

 ya think if i print this and take it to radio shack they could help me figure it all out and the parts?

josh1001 (author)rpvanpatt2010-02-22

well the triangular thing is a KA393 comparator and it is a small ic with 8 pins to find out the pin selections go to and type in KA393. You can use an enhancement mode p channel mosfet of our choice as the gate. To see how the comparators work check out

rpvanpatt (author)josh10012010-02-11

 nevermind I got it, but I dont quite understand it all, can ya explain the symbols a bit? sorry ...ive never made some one, or bought stuff for it neither

rpvanpatt (author)2010-02-09

Hey,  I like the idea, I am looking to find specific answers for a particular setup I am trying to put together.  Could you help me?  just message me thanks!

josh1001 (author)rpvanpatt2010-02-09

i'll help u, what do u need?

rpvanpatt (author)josh10012010-02-09

I am trying to create a mini solar panel battery pack/charger.  I have a mini solar panel that I would like to try to connect to a 9.6v nicd battery (2 if possible).  Also, I was thinking of putting an LED indicator that will let me know when the battery is charging/charged.  I have a drawing I am not sure if it will work or not, if so I dont know the particular parts that I would need.  Any help would be great, thanks

josh1001 (author)rpvanpatt2010-02-10

 OK I was working on a circuit to charge a 12 volt battery. Its a charge controller I designed I guess I can tweak it to meet your demands

nfarrow (author)2009-11-16

So what if I need a 12v charge controller for something like solar? What would you recommend?

josh1001 (author)nfarrow2009-11-16

I will upload a schematic too show u how to get it done. How much watts is this solar panel?

nfarrow (author)josh10012009-11-16

It’s just a 5watt solar but would like to be able to add more later on since I am working on refurbishing some old car batteries. I have about 12 now and I was told that I should hook them up in parallel.

josh1001 (author)nfarrow2009-11-18

Sorry im taking a bit of time, im testing and trying to improve circuit. I should have it soon for you. Is 30W of power ok?

nfarrow (author)josh10012009-11-18

No clue, I guess that will work. thanks :)

josh1001 (author)nfarrow2009-11-18


mathews (author)2009-10-19

You may want to place a 330R resistor in series with the the LED output.

Also, why do you have two zener diodes in your circuit, you only need one, and the other input should be a potential divider made up of two resistors. That way if it is over 5v, it will trigger on, and if not it will be off.

josh1001 (author)mathews2009-10-20

thanks for the tips, i tried the voltage divider circuit and it works. 
Question isn't the comparator sinking the led current ? 
What i mean is the led isn't a big load that would damage the IC, but in future i would put in a resistor just to be safe.

inventorjack (author)2009-10-19

You should explain a bit more how the circuit works. If I didn't already understand comparators, zeners and the like, this instructable would be totally greek to me. Perhaps explaining why you chose those specific component values and what the components actually do would be helpful to those less experienced in electronics.

josh1001 (author)inventorjack2009-10-20

This is my first time posting  an instructable, and it did realise that i could have change some things. By the way you are right abotu the lack of in fromation in the instructable, I'll update it soon.

inventorjack (author)josh10012009-10-20

Don't get me wrong. Not saying it's bad, just needs a bit more info.

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