3.5V to 12V circuit design

[tmarsh94]

I need to power a 12V DC relay which activates the A/C compressor clutch in my car, using odd voltage outputs from the climate control unit. The A/C output wire from the climate control unit is supplied +3.5V when the A/C is turned ON, and is supplied +1.2V with the A/C turned OFF. I need the +3.5V to somehow be turned into +12V to power the relay for the A/C clutch when the A/C is turned ON, without the +1.2V powering the relay when the A/C is turned off. I have basic knowledge of electronics, any help is appreciated!

[pebe]

This is a fairly simple way of doing it. The 555 timer is used as a level detector in this instance. Normally, the output goes high when Pin2 goes below 1/3 of the supply voltage, and goes low when Pin6 goes higher than 2/3 of the supply voltage.

Fitting the 3.3V zener diode to Pin 5 changes the level settings, so that the relay will switch on when the input goes above 3.3V and switches off when the input goes below 1.65V

I hope you find that useful. If you have any questions, please ask.

[tmarsh94]

Thank you so much for the help!

[tmarsh94]

Okay one question, could you possibly list the parts I need to do build this? Theres different wattages for the resistors and diodes, not sure what to get.

[pebe]

OK. Here is a parts list.

R1 does not pass any current so can be any wattage from 1/10watt upwards. Its purpose is to prevent possible damage to the 555 inputs due to spikes picked up on the input leads. Its value is not critical so it can be anything from 10K to 100K.

D1 can be any small signal diode such as a 1N4148.

The 3.3V zener can be a 500mw type such as BZX55C3V3.

IC is an LM555 timer (you may want to use an 8pin socket for it).

I am not sure if the 12V relay is already fitted in your car. If not, you need a 12V SPDT. That is with Single Pole Double Throw (or Change-over) contacts. You could also use a 12V SPST, although there are not so many single throw relays available.

I would suggest assembling on perfboard or stripboard.

I hope that helps.

[carkifelek]

As you can see in this circuit diagram, the 555timer acts as a buffer as well as a level translator for your output signal which is being fed to the relay. One important thing to be noted here is that general the relays used for A/C switching require a large amount of current. Be sure to check what the input signal current is required by that particular relay and whether or not the 555timer IC you’ll be using would be able to easily provide the required output current.

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