SN754410 Quadruple Half H Driver - what signal to activate ?

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.

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.

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.

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

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

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.

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.