More efficient circuit than using a relay?

Question

I've built a circuit that uses a 24 hour rotary plug in timer. [24 hour timer][1]

I cannibalised it to be part of a circuit I don't want turning on while I'm asleep. I have a sump pump under my house and the lowest part of the house is directly under my bedroom.

I've set the pins on the timer so it won't come on after 10PM and before 7AM. The output of this timer is 230 v AC. I'm using the output to drive a 230 v AC relay, and the relay switches 9 v DC to complete a circuit to operate a sump pump. The circuit times how long the sump pump is on for, and the pump can be triggered by a push button in the house.

So as a block it works as such: 24 hour timer -> relay -> timing circuit -> sump pump.

What would be a better solution than using a 230 volt relay to simply act as a switch to operate 9 volts DC?

I've already purchased the relay and used it. They now cost $47.90 at my local electronics retailer. So it's a pretty expensive way to switch 9 volts DC.

Answer 1

One solution would be to plug an inexpensive (possibly one you have lying around) AC adapter into the timer and use an inexpensive DC-coil relay to switch the 9V. The adapter provides the galvanic isolation and allows you to use a very inexpensive DC-coil relay.

Answer 2

So how good is your electronic knowledge?

How much current can the output of the timer at 230 V ac handle? Maybe you could use them directly to drive the power supply of the pump setup? The downside would be that the pump could only be operated by the timer.

An alternative would be to get rid of the timer and get an microcontroller with wifi or an RTC for the timing (maybe the onboard crystal of the microcontroller will be accurate enough for your timing) and use a transistor for the switching of the pump timing circuit. This could then be used to either controll a small relais or a transistor. Depending on the circuit of the pump the transistor should be isolated from the power supply of the microcontroller. For example an OPTOMOS or an Mosfet with an optocoupler or an optocoupler with the ability to directly drive a gate of a mosfet. The transistor and realis has to been chosen by voltage ( you said 9 V) and current and if you use a transistor for higher currents it should be cooled depending on its thermal specs. Since this is a signal contact i dont think it has to handle much current but better to measure it. If you use a transitor with a mosfet you have to get some sort of supply voltage from the pump or something like this and have to be sure it can be taken for driving the transistors gate. The easiest way would be the microcontroller with an 12 V or so relais or an optomos. Or you could simply trade you 230 V Relais in for some wich is driven by 230 V and switching maybe something like 12 V xx A ? If you dont want to tinker around you could use finished solutions... You even could use a smart relais. But keep in mind the 230 V AC line is dangerous and should be handled carefully with knowledge so you don't electrocute yourself or others and don't burn the house down.

Answer 3

24VDC is an industry standard for low voltages and relays, so go for that. In this case a relay which works on 24VDC coil voltage. There is plenty of such if you just search for "24VDC timer relay". That is:

• Get a 230VAC to 24VDC regulator - it only needs to be able to deliver the tiny bit of current needed to drive a relay coil.
• Get a 24VDC timer relay. Have it switching your 9VDC signal.
• As a rule of thumb, always place a flyback diode across the relay coil - especially if you got some manner of cheap voltage regulator of unknown quality. A plain 1N400x diode is good for that, for example 1N4001.