2N3904 as a switch

Thread Starter


Joined Feb 20, 2013
Probably a easy question....

Short my low voltage Home Automation module "Sensor" pin to "Ground" when motion is detected, using the least amount of parts, as space is limited.
- Home Automation Low voltage module pin-out.
----- Terminal 1 = 5VDC (for sensor use)
----- Terminal 2 = Ground (for sensor use)
----- Terminal 3 = Sensor Input (waiting for ground)
----- When I short Terminal 1 to Terminal 3, the HA module is activated and updates -----my database.

I have a DSN-FIR800 motion sensor.
- The sensor output = 5VDC when waiting for movement.
- The sensor output = 1.5VDC when motion is detected.
- Motion Sensor power is supplied by Home Automation terminals
----- Sensor Terminal 1 = 5VDC supply (VCC)
----- Sensor Terminal 2 = Sensor Output
----- Sensor Terminal 3 = Sensor Ground

My Home automation module requires that the sensor output be GROUND to be activated.

My thoughts were to use a 2N3904 NPN transistor (have a bunch of these) to short the HA module's GROUND terminal to the HA modules Sensor input terminal. That would work if when the motion sensor produced 5VDC when motion was detected and not 1.5VDC.

I thought about using a voltage divider circuit (10K pot with center tap) and adjusting the sensor output to "0" volts when it is active, which would be about 3.5VDC when waiting for motion. I really need to somehow invert the sensor output which would be used to activate the 2N3904 to supply the needed GROUND. I remember that there used to be a chip in the old 74xxxx series chips that was an inverter type chip, but as mentioned space is an issue. There has to be an easier way.

Your thoughts and suggestions. Thanks in advance for any assistance.
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Joined Apr 11, 2010
Do you have enough space for another 2N3904? By daisy chaining two transistors, the first is used as an inverter.

You’ll need a couple extra resistors, too. One on the base of the second transistor. Another is used as a pull-up resistor between the collector of the inverting resistor and Vcc.


Joined Mar 14, 2008
Below is the LTspice simulation of a circuit that should do what you want.
It uses a second transistor to invert the signal so that Out is at ground (0V) when In is at 1.5V.
D1 provides additional forward-drop at the input so Q1 turns off when the input drops to 1.5V.

(R_Test is just to show the operation of the output and is not used in the real circuit.)

It requires six parts.
Is that too many?

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Thread Starter


Joined Feb 20, 2013
After looking closely at the DSN-FIR800, I noticed that there is a H-L solder lead. I scratched out the lead and shorted the H lead and shorted the L lead, the sensor now outputs the proper signal when activated. Just needed to put on my reading glasses. Although the circuit you gave me will work perfectly on another project that I have on my work bench
Thanks again,