Challenge Circuit 6

Reed Switch Circuit

Solution and Practical Results

In this post, I’ll show one of the ways of solving Challenge Circuit 6: Reed Switch Circuit. There are 3 questions in this challenge circuit and we will discuss each question one by one.

Challenge Circuit 6 details


We have a cylindrical reed switch that has a normally open (NO) contact and a switching current rating of 1A. Any higher current could either burn the contacts or weld them shut so the reed switch is always on, making it unusable. Here are the questions

Q 1: We want to operate a 24V 25W incandescent lamp. How much current will it draw at the rated voltage?

Challenge Circuits 6-reduced

The lamp will draw 1.04 A

Q 2: Based on the previous response, if the lamp and the reed switch were connected as shown in the circuit, could it damage the reed switch?


From the calculation of the previous question, we found that the lamp will consume more than 1A to operate at the rated voltage. Since the rated switching current of the reed switch is 1A, any higher current could damage the contacts. So the response to this question is Yes, it could damage the reed switch. Yes, it could damage the reed switch.

Yes it could damage the reed switch

Q 3: If we needed to use an interface relay between the reed switch and lamp, what would be the right way to do it?

Circuit 1
Circuit 3
Circuit 2
Circuit 4

We want to control the lamp with the reed switch, via the interface relay. Since the interface relay is 12V and the lamp is 24V, we will need two different power supplies. 

We want the reed switch to activate the relay, so it will need to be connected in series with the relay coil. The relay coil is rated at 12V, which means we will use the 12V supply for the control side of the circuit. 

The relay contact has much higher rating than the reed switch, and we want use that to control the load, so the load will be connected in series with the relay contact. As discussed earlier, the rated voltage of the lamp is 24V, so we will use 24V for the power side of the circuit.

Circuit 1 is the best way to connect in this scenario

Circuit Connection Demonstration

Let’s connect Circuit 1 to see how it works.

Here is a video demonstration of the circuit and its measurement.


Most sensitive input devices like reed switches and other proximity switches are not designed to power a demanding load like an incandescent lamp or a motor. They are more suitable as an input for smart devices like a programmable relay, PLC, Arduino, etc. These smart devices can then control the load, either directly (if possible) or via a relay or a contactor.

In our case, we used an interface relay, which is not a smart device, but a simple example of an intermediate device that could control a higher demanding load.

If you would like to discuss other relevant points about the reed switches, please share them in the comments so our readers and students can benefit from your input.

Thanks for reading.

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Husnen Rupani

I help electrical training organisations increase learner engagement by designing innovative training equipment. I have a saying "Electricity - you cannot see, you cannot hear it, but by the time you feel it, it may be too late." My main aim is to turn this black magic that we call electricity into something that people can understand.