# Replace mechanical switch with relay

I have the following component controlling a garage door. I had hoped to wire it directly to my smart home system, which has outputs of 24V 500mA DC. (Output Module)

Based on the following schematic and with my limited knowledge of electronics, I gather that I cannot directly control this circuit. I have no further specifications.

I wish to replace the (non-existant*) pushbuttons T1 and T2 and control them using the relays in my existing system. At the same time, I would prefer something small enough to physically fit in control unit case for the garage door, but I can install a separate box if necessary.

What would be the simplest, most stable way to control this with the 24V outputs from my smarthome system? Preferably without requiring SMD components or a particularly steady soldering hand.

* I only have the circuit, there are no actual buttons attached to the module. The T1 and T2 in the schematic are the intended use of the module - to add two external physical buttons.

• Are these Up Down momentary switches? Is it ok to latch them. Or will you pulse each port to open/close? using phone wire to connect from existing relays. – Sunnyskyguy EE75 Nov 30 '17 at 21:41
• @TonyStewart.EEsince'75: I elaborated a bit. The buttons are not included. There only two leads and a common ground. – Troels Larsen Nov 30 '17 at 21:45
• YOu might get away with opto-couplers for this too if space is a concern. But if you are not the soldering kind..maybe not. – Trevor_G Nov 30 '17 at 22:04
• @Trevor: I can solder, I just didn't want a flimsy solution that will break for be too sensitive to moisture as it will be placed in a gargage with varying temperatures. – Troels Larsen Dec 1 '17 at 9:14
• @Trevor: I will check the next time I take it apart. Thanks! – Troels Larsen Dec 1 '17 at 12:58

You say the push buttons are non-existent? I'm assuming they are rubber/graphite buttons, which work just like push buttons. I'm also assuming that the buttons pass very small currents and voltages. You want to measure to make sure.

In either case, yes, you can simply replace them with mechanical relays. Of course you want to use relays which work at 24V coil voltage. Size wise, reed relays are pretty small, but you can also find SMD relays that are small that you could solder without issues as well. The diode is to protect your output if DC. If AC, you should get a relay meant for AC coil voltage instead of DC.

simulate this circuit – Schematic created using CircuitLab

• I elaborated my question a bit in terms of the non-existant part. The buttons are not included. I will measure the circuit, but I think it is either 3.3V or 5V. I will look into finding a suitable reed relay. – Troels Larsen Nov 30 '17 at 21:49
• You probably don't want that diode. 24V in a home-control context is usually AC. It's coming from the thermostat transformer. – Harper Nov 30 '17 at 23:24
• @harper what makes you think its that? OP has posted nothing regarding it. – Passerby Nov 30 '17 at 23:34
• Because 24V is no accident, it's pretty much the standard low voltage commonly in use in homes and businesses, e.g. HVAC, doorbells, relay controlled lighting, and all of it is AC and comes out of the standard HVAC parts bin. $10 40VA transformers that are listed, labeled and packaged for direct installation e.g. Built into a junction box cover, various RIB relays that mount in a knockout,$12 30A motor rated contactors, etc. etc. if they were brewing a DC system from scratch, they'd go 12V for interoperability with lots of auto/RV stuff, or 36V for more power. – Harper Nov 30 '17 at 23:56
• That doesn't say for sure it's AC, but it's how I'd bet. – Harper Dec 1 '17 at 0:05

Just get automotive-tier control relays, Mouser has them for about $3 each. The ones I use have blade connections, no soldering required. They're about a 1 inch cube, give or take. 24V (AC or DC) is a good choice because it allows use of cheap, plentiful, listed, legal controls intended for North American style HVAC, such as$12 air conditioning contactors.

• It is DC, sorry. I thought that was the norm. I'm in Denmark if that explains anthing. (230V AC). – Troels Larsen Dec 1 '17 at 9:00
• Same advice, then. – Harper Dec 2 '17 at 0:42