# Create SPDT relay from two SPST solid state relays

I'm trying to use two solid state SPST relays (P/N SAI4003D) to act like an SPDT relay. Ideally both will be controlled by the same signal wire. The relays state that they are "on" at 0 volts, so my idea was to use the arduino io to set them either input/output at low or input/output at high to turn them on.

I'm a little unsure if this will work since the relays are rated 3-30v so I don't know how much voltage will go to the other relay.

simulate this circuit – Schematic created using CircuitLab

For reference, It should act like this:

simulate this circuit

• Part number for your relays? Post a link in your question and not in the comments so that all the info required is in one place. Jul 5, 2016 at 21:45
• Are these SST relays? Magnetic? Please see electronics.stackexchange.com/help/how-to-ask for further guidance on asking questions. Jul 5, 2016 at 22:51
• laptop2d - it's stated in the first sentence. Jul 6, 2016 at 3:09

Figure 1. The datasheetless SSR.

You haven't posted a link to the SSR datasheet. I found the image shown in Figure 1 so we know it will work with 3 V DC but don't know what current it will draw. You need to hook one up to a 5 V supply and measure the current in on pin 3. Then compare that with your micro's output specification. Typically 10 to 20 mA should be OK. Be careful to check what the output voltage will be at that current - the higher the current the more the output voltage gets pulled away from the supply rail.

SPDT SSR

simulate this circuit – Schematic created using CircuitLab

Figure 2. A design worth trying.

How it works:

• When GPout is low D1 is turned on and SSR1 will conduct. LAMP1 will turn on.
• When GPout is high D1 will turn off, etc., D2 will turn on and SSR2 will conduct. LAMP1 will turn off and LAMP2 will turn on.

There is an interesting third state.

• If GPout is disabled (tri-stated or floating) both D1 and D2 will pass some current. According to the datasheet printed on the face of the device (it's all we've got) both SSR1 and SSR2 will fail to turn on and LAMP1 and 2 will turn off. At 3.3 V this should be quite reliable. At 5 V you might find that one or both stay on as there would be about 2.5 V across each one and 3 V DC would be a maximum for the minimum turn-on voltage (if you can follow that).

I'm trying to use two solid state SPST relays (P/N SAI4003D) to act like an SPDT relay.

Requirement met.

Ideally both will be controlled by the same signal wire.

Requirement met.

The relays state that they are "on" at 0 volts,

I didn't find that information and don't know what it means.

Looking back on your question I think this was what you were trying to draw in your schematic. I couldn't make sense of it when I first read it. I think you were on the right track - although a little lacking in elegance? ;^)

• Just put it together and it works as expected! Jul 11, 2016 at 2:22

You just need to use two output pins from the Arduino, and keep one high while the other is low and vice versa. Depending on how much current they require, you may need to add an NPN transistor driver, there are dozens of answers on the forum re that sort of thing. Since they are solid state, you won't need suppression diodes like you would for conventional relays.

• I'd like to use just one pin if possible as I'm short on IO slots Jul 5, 2016 at 21:41
• @kbickar Then put an inverter (such as a 74HC04 or 74LS04) in one of the lines to invert the signal. You can get the latter at RadioShack. Jul 5, 2016 at 21:58