power supply - Why is this reed switch / solenoid circuit not working?

The (normally open) reed switch should close by waving a magnet across it, completing the circuit. The 6v solenoid lock should pull inwards allowing the door to be opened. When the magnet is removed the reed switch should open.

So my issue is the solenoid pin isn't being pulled in when 9 Volts is applied. It only pulls in when pressed three-quarters of the way in. This made me suspect the voltage wasn't right but tested showing 9V. I tried two others batteries with the same results. I also tried putting two 9V batteries in series to make 18V, but the solenoid would only operate if pin was pushed in half way. ($%£!!!) I also noticed on both the 9V and 18V that the reed switch would get stuck in the closed position.

Any ideas would be greatly and deeply appreciated.

Answer

It sounds like you have two problems.

First, reed switches are delicate and shouldn't be used to switch a lot of current. When the switch opens (and, to a lesser extent, closes) a small arc is generated between the contacts. This can cause

1. The contacts to eventually weld together, so that it won't open correctly, or


2. A buildup of non-conductive junk between the contacts, preventing it from closing correctly!

You never know which failure mode will happen first. Even before it finally fails, it may become more and more resistive as the contacts degrade.

To solve the problem, you should use the reed switch to signal some other, less delicate, device. Then, this device should switch power to the solenoid. Common solutions would be either a MOSFET or a relay.

Here is a simplified MOSFET example:

^{simulate this circuit – Schematic created using CircuitLab}

It includes a pull-down resistor and a flyback diode (sometimes called "anti-kick diode"). If you like, please see this answer for why they are required.

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The second problem is that you need to provide enough power to your solenoid. It sounds like yours is rated for 6V.

If you are giving it the rated voltage and it isn't working, then you probably aren't able to supply enough current. 9V batteries have a high internal resistance, and so can't provide much current. You could put 4 AA batteries in series to get 6V and it could supply a lot more.

By the way, solenoids require a lot of current. AA batteries might not be sufficient. Do you know the current specification for your solenoid? You may need a dedicated power supply.

By the way, don't just increase the voltage! If you exceed the rated voltage you can cause failures in the solenoid.