Help with solid state SPDT switch using PNP transistors, to switch a Hall signal to an H-Bridge

Thread Starter

Hamlet

Joined Jun 10, 2015
376
I'm trying to build a discrete SPDT switch using transistors. I have a Latching Hall Sensor
with an open-collector output. When the Hall switches on and off, I want to create a POSITIVE voltage/
current signal (5v? 6v? 12?) that is routed to either of the two triggers of a H-bridge, but not simultaneously, obviously.

I did a search here first, and found the following schematic, (see below)
but it doesn't work with the parts I used. It sorta works, but the LEDs that I used
for loads didn't go off completely. I plugged a PNP BJT transistor into the schema as well,
but had poor results. We can think of the 2n3904 as being the open-collector of my hall sensor...
Rload1 and aRload2 are routed to my H-bridge (INPA/INPB, bottom) for forward/reverse.

Maybe I'm going about this all wrong, I need some help.

1610326860248.png
https://forum.allaboutcircuits.com/threads/how-to-emulate-a-spdt-relay.119279/post-946781

I'm using an H-Bridge like this:
1610328134136.png
 
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crutschow

Joined Mar 14, 2008
25,980
All you need to reverse the input signal polarity is connect INPB to the collector Q5 in your bridge schematic, plus reduce the value of Q5's collector 10k resistor to 2k, and increase the value of Q6's base resistor from 2.2k to 100k.

With those changes, the bridge will apply a positive voltage to the B side of the load when INPA is low, and a positive voltage to the A side of the load when INPA is high.

With that simple circuit, there will be a crossover current spike when the bridge changes state, but that should not be a problem if the switching rate is low.
A small resistor in series with the power to the bridge will minimize the value of the spike (what is the bridge load current?).
 

Thread Starter

Hamlet

Joined Jun 10, 2015
376
With that simple circuit, there will be a crossover current spike when the bridge changes state, but that should not be a problem if the switching rate is low.
A small resistor in series with the power to the bridge will minimize the value of the spike (what is the bridge load current?).
Thank you. This answers/addresses half my question, and solves a problem that I didn't know I had. To answer your questions, the switching freq. is 2Hz to 100Hz. Current is 2A to perhaps 5A.

My unanswered question is thus: Is my first schematic valid for developing a viable SPDT switch as operated from an open-collector? Can it be implimented with PNP transistors?
 

Thread Starter

Hamlet

Joined Jun 10, 2015
376
1) H-Bridge may controlling either a small motor, or specifically, a linear motion solenoid with a magnet for a plunger.

2) Small motor's and/or solenoids are used to activate motion in kinetic sculptures.
 

sghioto

Joined Dec 31, 2017
1,796
This should work but need to know the max voltage the Hall sensor is rated at.
I don't see why you would need this though.
1610395953049.png
EDIT: Schematic corrected
 
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Thread Starter

Hamlet

Joined Jun 10, 2015
376
That was my point especially if using transistors.
What's wrong with just using the circuit in post #12?
Nothing wrong with your circuit as far as I can see. Thank you.

I have some complimentary arduino h-bridge L298N modules on order that utilize
two positive (5v) signals to enable switching that I'd like to try without the microcontroler.
 

Thread Starter

Hamlet

Joined Jun 10, 2015
376
The L298N is rated at 2 amps will that work?
You mentioned up to 5 amps in post #4.
Yes, thank you for the heads up; the L298N work for small applications, otherwise, schema in post #12 for higher currents.

Do you know of any cmos logic chips that can impliment the SPDT switch?
 

Thread Starter

Hamlet

Joined Jun 10, 2015
376
Yes, but again why? A SPDT switch has to be in one position or the other. When one input is high the other is always low.
With the L298N that can be accomplished with one NPN transistor.

View attachment 227497
Excellent! I was unaware that was an option, I can see it now, thank you!

As my most pressing questions have been met, I'll be reaching for a copper clad pcb.

Thanks to all for the help, and I will return if I get stuck.
 
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