My garage opener has a small courtesy light and I thought I could use the voltage from this light to switch a relay to switch larger internal and external mains operated lights. The voltage going to the LED PCB when the garage is opening is 24V DC. When the courtesy light switches off the voltage to the PCB drops down to 10V DC. I tried using a 24v Relay which switches on fine but does not disengage when the voltage drops to 10V because the holding voltage of the relay coil is something like 6v. I tried using resistors to drop the 24v to the lowest voltage that switches on the relay (18V). This is turn dropped the 10V to 7V which still kept the relay switched on,

Anyone has any ideas? I can't find 24v relays that have a holding voltage higher than 7v. Thanks in advance for any help you can provide.

 

Couple options:

1.) Feed the courtesy light voltage to a comparator via a divider network, and set your comparator reference to above 10V. Drive a MOSFET with the comparator and switch your relay using the MOSFET.
2.) Elevate the relay ground by 10V using a resistor network.
3.) Use a P-Channel MOSFET with Vgsth > 14V. Drive your relay using this. Logic will be inverted but just swap NO for NC or vice versa depending on your application.

Remember flywheel diode in all cases.

 

Jterblanche, I can't thank you enough. I have been away from the electronics scene for a while. Yes a transistor should do the job.

I didn't quite understand solution no. 2. Does that mean putting the resistors on the negative feed of the relay?

 

Correct. Select resistor values such that your relay gets sufficient current.

 

You're a champ!