I'm looking to use my Spark Core to control my door buzzer in my apartment. I'd like to identify the correct parts I need to build the required circuit, but this seems to be dependent on the voltage and current type that the buzzer system uses. I have been referencing this question: to try and identify the proper relay and transistor needed. Here is the schematic for my buzzer system (My apartment unit has model 3404 inside it with LISTEN, TALK, and DOOR buttons):


According to this similar example, I need to find a relay with a coil voltage equal to that of the door buzzer system. Also, as the Spark core provides 3.3 VDC, I believe I would need to amplify this using a transistor. From the schematic, can you help me identify what the voltage and type the system uses, and also what types of relay and transistor I would need for the circuit?

Many thanks!

  • 1
    \$\begingroup\$ The schematic you gave gives hardly any information at all. You can use a general purpose switching transistor. To determine your voltages, I'd recommend you get in there with a volt-meter and measure it. \$\endgroup\$ – horta Sep 23 '14 at 17:44
  • \$\begingroup\$ @horta thanks for the answer I did find this link to the intercom amplifier: alphacommunications.com/aps802. There it lists the voltage at 16VAC. Does this mean I would need a 16v relay and a DC to AC converter? \$\endgroup\$ – Jeff E Sep 23 '14 at 18:53
  • \$\begingroup\$ No, what you'll want is a 3.3 volt coil on the relay which switches the 16VAC circuit. The transistor amplifies the current output of the spark core general purpose IO pins so that the coil has enough current to switch the AC circuit. If you describe exactly what you're after I'll write up an answer with a circuit so you can see how it will operate. Are you doing anything with the other buttons on your module? (talk, open-door) \$\endgroup\$ – horta Sep 23 '14 at 19:43
  • \$\begingroup\$ And from that link it also states that it takes 10VA: "16VAC, 10VA". So you'll want the relay capable of handling 10/16 = 0.625 Amps. \$\endgroup\$ – horta Sep 23 '14 at 19:49
  • \$\begingroup\$ A full answer with a circuit would be amazing! What I'm going after is to be able to send a text message through Twilio that will complete the buzzer circuit and open my front door. Essentially, the text will replace the functionality of pressing the DOOR button and open the door. I'm thinking that the Spark Core will turn a pin to HIGH when the text is sent, closing the relay and completing the door buzzer circuit. \$\endgroup\$ – Jeff E Sep 23 '14 at 20:34


simulate this circuit – Schematic created using CircuitLab

As noted in comments, the transistor can be any general purpose switching transistor. The transistor must be able to handle the current necessary to activate the relay coil. The relay coil must be able to be switched on by 3.3V. The relay must be capable of switching a 16 VAC load and be able to handle 0.625 Amps.

Note that the AC ground likely won't be connected to the Spark Core's ground because there shouldn't be a need and it could cause issues if you hook things up wrong.

A high signal from the Spark Core will activate the relay and activate the Buzzer. A low signal would block current in Q1 which would prevent RLY1 from activating which would prevent the buzzer from activating.

  • \$\begingroup\$ Thanks @horta for the awesome circuit and description. For the specifications on the relay, I know there are: Voltage, Coil resistance, Contact Rating, and Nominal Current. I assume the Contact Rating would need to be > 0.625 Amps of the relay. What are the other specific specs I would need to look for that correspond to the 3.3V coil and the 16VAC load? The voltage rating of the relay needs to be exactly 3.3V? \$\endgroup\$ – Jeff E Sep 23 '14 at 21:08
  • \$\begingroup\$ This is the relay I have now, would it work for this purpose? radioshack.com/product/index.jsp?productId=2062478 \$\endgroup\$ – Jeff E Sep 23 '14 at 21:28
  • \$\begingroup\$ @JeffE Your voltage rating will be for the output side (i.e. 16VAC). The coil resistance will determine the amount of current that will flow through the transistor. As an estimate you can do 3.3/coil_resistance = I_thru_Q1. You're correct on the contact ratings. You should either have a current or voltage rating for the coil though as well. You need to know how much current it takes to activate the relay. The voltage rating should be greater than 3.3 V on the input side and greater than 16V on the output side. \$\endgroup\$ – horta Sep 23 '14 at 21:29
  • \$\begingroup\$ @JeffE No, it won't work for your application, it's rated for 5 V input. And from the datasheet here: datasheet.octopart.com/… you have to have 3.75 V to activate it. \$\endgroup\$ – horta Sep 23 '14 at 21:32
  • \$\begingroup\$ @JeffE If you have a higher than 3.3 V power rail, you could use that to operate it. If your entire board runs on 3.3, it'd probably be easier to get a different relay. \$\endgroup\$ – horta Sep 23 '14 at 21:33

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