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I am trying to bypass the pushbutton of an external device (an LTE Hotspot) and switch it on using a Raspberry Pi. The pushbutton pulls down 1.8V for 2 seconds and then the external device powers on. I'd like to do this using an RPi but since it runs at 3.3V I am hesitant to tie it directly to one of the GPIOs as I don't know if the device is 3v3 tolerant. Ideally, I could have used a logic shifter but the external device does not expose the Vdd.

Is there a way to perform a unidirectional level shifting with minimum components? Any ideas are welcomed.

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    \$\begingroup\$ Why not use an NFET to pull it down with an active high signal from the pi? The usual bi-directional FET circuit will probably work too, if you have one on a little postage stamp board already. \$\endgroup\$ – Chris Stratton Jul 2 at 18:43
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So, from your description I assume there already is a pull-up resistor of some sorts to 1.8 V and the push button pulls some input of the router down to ground. In that case, you can just add a Schottky(!) diode. It will allow the Raspberry Pi to pull the router input down, like the push button does; and it'll prevent accidentally injecting 3.3 V into the router:

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ I have implemented this and it works as suggested. I am accepting this answer as it the minimum number of components \$\endgroup\$ – jiko Jul 5 at 19:58
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Here Q1 acts as a level shifter, R2 and R1 form a voltage divider that produces about 1.8V at the base, so when the input voltage is pulled below about 1.2V base current flows and allows the collector to pull down the output from +3.3V. R3 is optional if you enable the pullup in the RPi.

schematic

simulate this circuit – Schematic created using CircuitLab

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