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Switching with transistor versus relay

jgrub

Element14 has a great YouTube vid on transistors, and I get that transistors are faster than relays and are better in low voltage situations. But I've been trying to custom build a motion sensor that remotely turns on the motor of an animatronic prop. It should be easy enough to activate that motor from the collector side of my transistor. 

 

But because the prop is a distance away (maybe 5-6 feet), it has it's own power supply and a separate ground. All I really need is something to close the circuit on the ground side of the prop motor. So it seems to me that two wires from the prop to a remote relay, NOT a transistor C/E connection, would make more sense. The activated motion sensor would send 3.3v to the relay. Then the relay connects the two wires together and that completes the motor circuit.

 

I've been trying to do this with a transistor switch for days and can not figure out what I should be connecting to the motor two wires.

 

Has anyone else tried to rig a simple remote switch like this? (Halloween props typically use foot pads that complete the circuit in a similar but highly mechanical way.)

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  • Hi Joe,

     

    Also, really a transistor is not a switch, but consists of semiconductor junctions that have switch-like behavour, within certain parameters, from a birds-eye view. From a closer view, that switch-like behaviour might or might not be suitable for the application.

    One thing that could affect the choice, is the motor requirements (current consumption). If that is low, then you may be fine using the transistor, but as you can see in Frank's circuit, a diode is also needed, because the transistor switch is fragile in some ways too. Also, motors demand more current when loaded, so that needs to be considered too.

     

    If the current consumption is high, then you may need to use a relay, but that may entail using the transistor too, because the 3.3V output from the motion sensor may not have the current drive to directly control the relay. It depends on the sensor output.

    So basically some more information is needed, or will need to be checked (motion sensor output capability, motor current requirement and voltage.

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  • Hi Joe,

     

    Also, really a transistor is not a switch, but consists of semiconductor junctions that have switch-like behavour, within certain parameters, from a birds-eye view. From a closer view, that switch-like behaviour might or might not be suitable for the application.

    One thing that could affect the choice, is the motor requirements (current consumption). If that is low, then you may be fine using the transistor, but as you can see in Frank's circuit, a diode is also needed, because the transistor switch is fragile in some ways too. Also, motors demand more current when loaded, so that needs to be considered too.

     

    If the current consumption is high, then you may need to use a relay, but that may entail using the transistor too, because the 3.3V output from the motion sensor may not have the current drive to directly control the relay. It depends on the sensor output.

    So basically some more information is needed, or will need to be checked (motion sensor output capability, motor current requirement and voltage.

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