I want to use an uno board to power a mechanized prop for a pinball machine mod I am working on. Controlling the prop with relays is no problem but I want to connect the uno's input to one of the flasher bulb circuits to trigger movement of the prop. The issue is that the flasher bulb circuit I want to use for input runs around 24v dc, high current, and will fry the uno if I don't drop voltage somehow. How can I drop voltage without using a regulator that may drain the current, such that the flasher bulb still functions without overloading the circuit? This should be possible since the uno input only requires a trickle of current.
Go for the optocoupler. It will save a lot of pain.
10 mA into the optocoupler - 2k2 resistor. Dissipates a quarter watt. If it is never on continuously, a quarter watt resistor would do, otherwise use a half watt.
Output of the optocoupler goes from Arduino input to ground, use the internal pull-up. That is a total of two components.
You said the light used 24V. The chip input rating for a '1' is from .6VCC to VCC+0.5V. For a 5V chip VCC that means a '1' is anything between 3V and 5.5V. To reduce 24V to something in between, we start with a desired voltage of (3+5.5)/2 or 4.25V. Nice and safe.
Also we don't want to draw too much current from the +24V, so we pick a safe value like .002A. For 19.75V (24v-4.25v) we'd need a resistor that would pass .002 amps, or from Ohm's Law 19.75v/.002A which gives us 9.785k. Closest standard value is 10k.
Now to get near 4.75v at .002A we need something around 2.375k. I don't like to use 5% values that much so instead of 2.4k, I went with 2.2k, a more common junk box value for me. That gives about 4.33V, close enough.
raschemmel:
It doesn't work well at all. The forum wants to format things it's way. I give up. We used to do this all the time on usenet.
