I want to connect an AC powered light to a device that is powered by 9 Volt DC current and has an LED light.?
I bought a reed relay and connected it. The reed relay needs 5 VDC to drive it. The way I had it connected made the large light flicker but not light completely. I had the relay coil wires connected to: the positive poll of the metronome's 9 volt battery and the negative lead of the metronome's LED.
Is there a better way to accomplish my goal of making the larger light flash to the designated beat of the metronome (other than using a reed relay)? Or is there a correct way to connect my reed relay to accomplish my goal? Or do I need a different reed relay? The reed relay is 5VDC voltage, 250ohms coil resistence, 0.5A at 125VAC contact rating, 20mA nominal current.
Answer:
I would have expected your reed relay at 20mA to at least be close to working... and it sounds like it was. It seems clear from your description that the circuit is trying to sink the LED as opposed to source it, and that's how you've described wiring it so far.
It is possible that the current limiting resistor for your LED was in the (-) side leg, and that could still be limiting the current on your reed relay.
You may have another situation that could cause problems, though. We don't know what was driving the LED, and it may NOT appreciate having to sink a 9V load. It could just be a small TTL IC that isn't expecting to sink more than 5V, so even if we get this working, you could damage the driver.
Operating on the assumption that your metronome is trying to sink LED current, I would suggest that you use a small PNP transistor to replace the LED. We'll add a bit more resistance to the current limiting.
(+) LED metronome output to transistor base
220 ohm resistor to your battery 9V (+)
other side of 220 ohm to the emitter of the transistor
one side of reed relay to transistor collector
other side of reed relay to (-) LED metronome output
That should drive the relay well for you.
BUT! It is also possible that the flash is too short to really light up an incandescent lamp properly. However, you now have a reed relay that will drive a number of LEDs of higher brightness for you.
I could tell you how to do it, but if your level of ability is using relays, I'm afraid you're on your own because you will have to build a small logic board.
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