I am wanting to modify an existing battery powered device, to switch it on and off using a raspberry pi. To do this I want to make a transistor relay circuit. However, I need to know the current of the circuit and it is here that I am running into problems.

I have tried measuring the current whilst connecting one probe to the battery and the other to the battery terminal as advised by a friend (I am a compete noob to electronics), but the device won't switch on.

I have tried to connect a resistor between the battery terminal and the battery, but the device briefly lights up, for just an instant and then shuts down.

I have tried measuring the current over the resistors on the board, but they are very small smd resistors and I can't get a reading.

Can anyone advise on the best way to measure the current?

I am using a fairly cheap digital multimeter. I have a few spare 100ohm resistors.

Many thanks

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    \$\begingroup\$ Look into the EEVblog uCurrent. \$\endgroup\$
    – Jeroen3
    Jun 8, 2020 at 11:12
  • \$\begingroup\$ Hi Miller, 'I have tried measuring the current over the resistors on the board' … Did you try measuring current or voltage? \$\endgroup\$
    – vu2nan
    Jun 8, 2020 at 11:22
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    \$\begingroup\$ In short, fix the following things in your question: 1) Add a picture of the device you are trying to modify 2) Add the name, product number, and function of the device 3) add why you think a relay+transistor is the right solution to your problem \$\endgroup\$
    – Joren Vaes
    Jun 8, 2020 at 11:37
  • \$\begingroup\$ Describe the device, describe the sort of batteries it uses and how long they last. This is a case where you may need to have some rough idea of the answer before you can pick the appropriate tool to measure it specifically. \$\endgroup\$ Jun 8, 2020 at 22:50

1 Answer 1


Do you really have to measure current? A P-Mosfet like a DMG2305UX has digital switching levels and can switch something like 4 Amps of current.

Personally, I'd just slap down a P-Mosfet between the battery positive and the device positive (Careful! Drain to device as these things have parasitic diodes) and only worry about current if it gets hot or turns to smoke.


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