Srd-05vdc-sl-c Dry contact? Bell timer?

Hi all,

Forgive my newbie question. I am trying to create a 'Dry contact' that will set a bell off on a QSM-624ET Public Address Amplifer. I have heard a relay must be used to do this, I have a srd-05vdc-sl-c wired up with pin8 to the S and 5V to + and GND to GND, the COM is suppose to be the middle output pin and the trigger the last output pin. May question is, is there actual voltage that is passed over these lines? I've connected up an LED and nothing comes out so it this achieving what im after?

Show us a good schematic of your circuit.
Show us a good image of your ‘actual’ wiring.
Give links to components.

Supplying a dry contact to a circuit means the contact is simply an open/closed switch. In the case of a relay, it is a contact which is either N.O. or N.C.


It is the load that wets the contact.

Your LED can be connected to the output of the Arduino (use a series dropping resistor).

The Code:

int relay_pin = 8;

void setup(){


void loop(){





Use the D1 or D2 circuit for output indication of relay status.

Thank you, Larry, I guess my question is do these 2 wires a 'dry contact' when NC? so my LED was just a test to see if its working or not (don't have my multimeter handy) these 2 wires apparently is all that is needed, when they are active (a circuit created) it sets off the bell

When the relay is not energized, the contact between C and N.O. is OPEN i.e. the resistance you will have between the two points is infinite ohms.

When the relay is energized, the contact between C and N.O. is CLOSED i.e. the resistance you will have between the two points is zero ohms.

@iHaag, your topic has been moved to a more suitable location on the forum. Introductory Tutorials is for tutorials that e.g. you write, not for questions. Feel free to write a tutorial once you have solved your problem :wink:

Who says this?

Hmm, the user manual is really no help here.

I very much doubt a relay is actually necessary as these inputs must be digital controls to a sound synthesiser.

I would suggest you start by testing with your multimeter across the two terminals. First measure the voltage and determine which is positive. Then switch to the 200 mA current range and test the terminals. This will emulate a switch being closed but will tell you exactly how much current it needs to switch. Switch down the current range until you get a suitable measurement.

If as I suspect, it is something like 5 or 10 Volts and no more than 20 mA, then an optocoupler will do the job perfectly and will be easier to use than a relay module.

Please get back to us with your findings.

So the voltage is applied to the coil of the relay so it will make a dry contact. I just tested it and seems to work fine. Thanks for all the help guys. Silly question, infinite ohms and zero ohms another term for dry contacts?

infinite ohms = OPEN contact

zero ohms = CLOSED contact

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