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Topic: how to hookup this relay? (Read 50172 times) previous topic - next topic

mckenzm

To be really clear, as some of these posts wander... See note 4) if you are short of pins.

1) The signal only drives the (gallium arsenide?) LED in the opto coupler, not the coil. Through a resistor.

2) The coil must have 5V @ ~75mA

3) There may be a flyback diode on the coil as well.

4) If you do not have a pin for the 5V additional VCC, it is intended for you to use the jumper pin itself.

5) But take care not to tie two differing supply voltages together, as you would when programming from a USB to serial adapter (as for a pro mini).

6) Even though it is a coil, and should act as a choke, your decoupling capacitors may not be enough to filter out droop or spikes. As well as it picking up any local interference by acting as an antenna.

7) Opto-isolation protects the microprocessor and the signal in case of trauma or failure, especially in automotive applications. The trauma would have to be severe for the switched voltage to affect anything upstream, and in those extreme cases opto-isolation may not help. (Crowbar, earthquake, car wreck). 

8) If the relay will not trip, and the output led is weak, check your pin is an OUTPUT pin, high in the DDR for that port. You may be able to get 3.3V as an input signal, but the coil still needs 5V, and this would be another case for 5V supply. 

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