SN754410 Quadruple Half H Driver - what signal to activate ?

Using Arduino General Electronics
#1

Hi
I am modifying a RC car. The new battery pack / motor is 9V, and the car receiver runs 5V.
I have added a 5V regulator to feed the receiver board, and all working perfectly.

Also added a SN754410NE Quadruple Half H Driver 16-Pin Plastic DIP so the output from the receiver can control the 9V to the motor.

All working except the SN754410NE

If I read examples on the net, it looks like the input pins ( pin 2 - 1A and pin 7 - 2A ) require a 5V+ input. But I am finding that the receiver outputs 5V+ to pin 2, and 0V to pin 7, but the motor does not turn.

I tried disconnecting the input pins ( pins 2 and 7 ) and then connecting the 5V+ from the regulator to either of these pins. Nothing happened.

But if I connect Ground to either pin, the motor spins perfectly as it should.

My only guess here could be that the SN754410NE has internal pullup resistors on the inputs ( pins 2 and 7 ), so the signals from the receiver, when input pin 2 = 5V+, is not grounding pin 7. And that the internal pull-up is working like an active input signal on pin 2 when I connect ground to pin 7.

Could anyone please tell me if I am understanding this correctly ?

If this is correct ( meaning that the input pins actually need a ground on the ‘opposite’ input pin to activate the motor ), could the problem be overcome by adding a pull-down resistor to the input pins ? Is this even possible if the SN754410NE has internal pull-ups ? And what size resistor to overcome the internal pull-up ? I am thinking along these lines as I would prefer to not have to modify the existing receiver, and to use only the 5V+ signals that I am already getting from it.

< addition >
Or would it be better to add a 2N3904 transistor, with SN754410NE pin 2 's 5V+ connected to a 10K resistor connected to 2N3904 Base, 2N3904 Emitter to Ground, and 2N3904 collector to SN754410NE pin 7. Thinking that if SN754410NE pin 2 is activated with 5V+, then the 2N3904 is activated to pull SN754410NE pin 7 down to ground. A second 2N3904 would be used to sink pin 2 when pin 7 is High.

#2

RC receivers output PWM signals as used by servos and ESCs, they cannot drive an
H-bridge directly.

The SN754410 inputs read HIGH reliably if left open circuit, yes.

Are you trying to replace an ESC with an H-bridge? In which case an Arduino to
monitor the receiver's output (using pulseIn) can then generate the full signals to
handle an H-bridge - here "PWM" is controlling the power, its not a RC PWM signal
which is quite different.

#3

Thanks for the reply MarkT

The RC receiver is a very cheap unit from a kid's toy. I have checked the output and am seeing a constant 4.7V+ on the line being driven, and 0V on the other line. ( switched around when RC controller changes direction ).

I suspect that the 0V that I am seeing is not 0V ( as in Ground ), but just no voltage, no ground, not connected to anything.

So I think the only options that I would have ( with my limited supplies and parts available ) would be to pull down the line that is at 0V with a resistor, or with a transistor which is activated by the high line.

Any advice welcome.

#4

Yes, transistor(s) as inverters would make sense if the receiver is only pulling up.

#5

to MarkT

Many Thanks for the input and guidance.

Feedback is that the transistors worked perfectly and the H-Bridge is not operating exactly as it should.

Now that I think that I grasp the idea of having the pins high and low ( as opposed to simply making one High ), I wonder if it would not be easier to simply have a pull down resistor connected to both pins 2 and 7, so they are Low by default, and each only requires a positive signal to activate ?

Regards

#6

DaveO:
Feedback is that the transistors worked perfectly and the H-Bridge is not operating exactly as it should.

I presume that should be "now", rather than "not"?

Now that I think that I grasp the idea of having the pins high and low ( as opposed to simply making one High ), I wonder if it would not be easier to simply have a pull down resistor connected to both pins 2 and 7, so they are Low by default, and each only requires a positive signal to activate ?

Regards

Not if they already have pull-ups holding them high, that would be fighting pull-downs.

#7

now noW nOW NOW !!
damn autotype.
At least I wasn’t trying to say ExpertS EXchange :slight_smile:

I hear your logic on the pull-up / pull-down resistors. So better to use a transistor to sink the opposite pin to ground only when required.

I have learnt so much from such a small project.

Thanks again for your help.

#8