Switching a relay when a DC current flows through a wire, independent of the amount of current. 


This is triggered by a fellow ham who tried to switch driving lights without intrusive modifications in a modern car. I do not know what his current results are, but at the time he was laboring with it, I thought I just quickly check one of my various current sensors I have laying around.

Considerations / Objectives

  • We need to measure DC not AC
  • We want the output signal TTL like to either directly connect to a microcomputer or drive a relay.
  • The output signal needs to be independent of the amount of current flowing. It should always show high even with the smallest currents. 
  • The supply wire to be measured must not be cut.


In my collection was only one sensor type which would achieve the objectives. It is a WCS1800 mounted on a small board with an op-amp for signal processing. See image below.

This little board delivers exactly what we need. The DOUT signal is either low or high. Low when idle and high when a current is flowing and with the little trimmer poti the threshold can be adjusted.

There is a red LED which lights up when the threshold is turned down to idle level. That is what we want, because then the signal is low. Any small current will now switch the signal to high and it can be used further, with whatever needs connecting. If one wants to switch 12V relay, a little amp stage is required to boost the output signal to the coil voltage (12V), otherwise a small 5V relay can be switched directly.

The AOUT is used for measurement of the current and is an analogue signal, which can directly be connected to a microprocessor like an Arduino.

Note: All the clip-on sensors I have were AC only, and mostly are. The WCS1800 does both but needs to be mounted in the correct direction. The arrow at the top of the housing shows the flow direction required for the DC current.

The image above shows idle voltage of the DOUT signal. The red led is lit, meaning the threshold is as low as the idle level.

Above image shows the output voltage of DOUT with a small current flowing.

NOTE: Supply voltage for the sensor module is 5V.

This is a really simple solution to sense DC current for simple On/Off applications. However, one needs to be able to feed the wire through the sensor. I do not have a clip-on DC sensor, but they do exist.