Switching with transistor versus relay

Hi Joe,

Can you post a simple schematic with the transistor part number and how you have wired it up?  For a simple example of a microcontroller controlling a DC motor with a transistor in a Halloween costume see this post:  LadyBug Girl and Rocket Boy .  The schematic with the motor is near the bottom of the post.  If a foot pad will be used for a switch then it is not clear that a relay or transistor is needed at all.

Frank

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.

Great input, guys! I worked on my diagrams this morning and here they are:

The left-hand image shows you my motion detector (with the project box drawn around it for placement of access holes). The sole purpose of the LED is to dimly light the interior of the box to show that the power is on. Hence the very high value resistor. Note that, at the bottom of the left-hand diagram, I have two wires coming out to the right-hand diagram. The left-hand model is my trigger (HC-SR501) and my switch (2N3904). I'm trying to use it like a simple relay to connect the right-hand circuit to ground.

The right-hand image is what's inside my Halloween prop (a hanging zombie). The motor is weighted so as to vibrate the prop. Then I have two LEDs for lighting up its eyes. It's powered by a 6vdc adapter that plugs into 120vac. The slide switch is SPDT so that I can turn it ON (constant on), OFF, or set it to get its completed connection (to ground) from the trigger/switch on the left. All components of both halves of this project appear to work fine. My issue is how to connect them to each other to just create a simple grounding -- or circuit completion -- on the right side. I have a shipment of relays coming today, but would still like to solve the transistor issue, simply because it's an obvious gap in my knowledge.

FRANK, I mentioned the foot pad just to indicate what these props usually use if they don't have an on-body sensor. My hope to replace the unreliable (and too small) foot pads with these little sensor boxes (left-hand diagram) that can simply plug into the foot pad socket of the prop and then be mounted strategically.

BTW, this is an ongoing project for a haunted house that my son creates every year. Hoping to come up with some cool monsters to help him expand that. 

Douglas,thanks for this. As I study the configuration, it looks a lot like what I already have (except for the protective diode, which I need to add), except that you've placed the transistor and resistor in the motor "assembly", whereas I've placed them in the sensor "assembly". But wiring looks essentially identical, i.e. current flows from the motor to the collector, while the emitter goes to the negative side of the power source (or ground).

As I'm very rusty at this, can you verify my observation? Or maybe I'm not seeing something else? Thanks!

That looks like a fun project.  I am away from home and pecking on my tablet but a couple of thoughts...

* you need the diode for sure

* it may be better to use a logic level MOSFET - the transistor amplifies current and I don’t know what the PIR can deliver and there is a 1k resistor.  I have always read PIR output with a microcontroller.

* a quick look at the 2N3904 in a TO-92 package indicates it is only good for 200 mA and your motor can draw 2 A.

* what current are your LEDs rated for?  Don’t think they will work on the prop side as shown but I am unable to mark your schematic up.  100k is going to make a very dim LED on the other side.

Hopefully shabaz or dougw can have another look otherwise I will look again when I get home

I just linked the first schematic that illustrates how a transistor can be hooked up to drive a motor. As fmilburn points out you need a transistor that can handle the load, maybe a TIP120 or a big FET like an IRL520 as Frank mentioned. If the power dissipated in the transistor is over about .3 Watts you will need a heatsink on the transistor.

Hi jgrub props and animatronics are so cool so I hope you get this going.

IMHO as this required on/off functionality is of a low periodicity I would opt for the relay approach as the quick and simple way of getting this working (and then being enjoyed). If you put a large enough relay in the thud of the contact switch over would surely add to the scare factor on a dark night? However if you are intrigued by the transistor approach then, sure, try them out. I would explore the FET/MOSFET route as many devices have low internal resistances, allow switching of large currents.

If you do go the MOSFET route remember that they are also transistors (transistors are not just the Bipolar Junction type) and as fmilburn states they need a diode to prevent any motor back EMF damaging the device.

Joe,

I would suspect that you need a more complete path for the Ibe current to flow.  At present the path from the PIR through the base-emitter junction is not complete.  There needs to be a path from the emitter to ground (common to the PIR output) in order for current to flow through the base-emitter junction, allowing current to flow through the collector-emitter junction.  By connecting the emitter of the transistor to the ground connected to the PIR sensor, you should be able to complete this path.

Good luck,

Gene

Hi Joe,

the big difference in your schematic and that of Doug was that you did not have the emitter of your transistor tied to the ground of the PIR control module. Without this connection the emitter base circuit would not be able to produce a current flow and the transistor could not turn on.

John

EDIT> Seems that I should have read Gene's post before I jumped in.

Shutting up now, with a red face.

John