Electronic – place a relay at the output pin of a 555-timer

ne555relay

I recently remember running across a bunch of articles on web about the 555-timer, it’s my first IC.

I’m getting more familiar with it, and I want to know run it in a-stable mode, with a relay between pin-3(Output) and ground. I will be using the relay to switch between two different LED arrays.

What concerns me is whether placing a relay directly between pin-3 and ground would cause a high current situation in the 555-timer.

Would it be necessary, and or possible, for me to put a resistor in series with the coil to limit current?

I’m just now getting experience with relays to, I know that they require a certain applied voltage to magnetize the coil to a point where it can effectively move the armature. I’m just not sure how this relates to KVL?

At the high state of pin-3 12V would be applied to the coil, I’m not quite sure, but I believe without a resistor, that would probably mean a lot of current would be sinked to pin-3. If I were to put a resistor in series with the coil of the relay, At high of pin-3, would the coil maintain the required voltage across it?

Best Answer

To elaborate on Peter Bennett's answer, here is the standard relay driver circuit I use. It removes the requirement that the 555 can supply the full current of the relay.

schematic

simulate this circuit – Schematic created using CircuitLab

Notes:

  • Relay power doesn't have to be the same power supply as the rest of the logic, but it can be if that is adequate voltage for the relay. Ensure the relay power supply voltage is adequate to close the relay.
  • Select Q1 for adequate relay drive current. For large relays, the 3904 may not be appropriate. Also ensure that the device will saturate. A darlington connection may be required for very large currents.
  • D1 is a flyback diode required to protect Q1 from voltage spikes caused by the relay coil.
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