Circuit Analysis of IR transmitter/receiver

[Baluncore]

Not meaning to butt in your great conversation - just a thought to consider here...

I didnt see bypass capacitors across motor or Vcc , maybe i just missed them...

You are not butting in, you are welcome. You see the things that I miss. I never seem to need bypass capacitors in my LTspice models.

Brushed motors are electrically noisy critters. Try a .1 or 1 uf right across the motor terminals, and a hundred or so across Vcc to circuit common.

True, they are noisy, but if you add parallel capacitance to the inductive motor then you need another inductor between the motor and the the PWM drive transistor. Don't forget you must have the parallel flyback diode over the inductive load to protect the driver transistor.

When I try to run the motor at a high duty cycle it keeps quitting. I imagine it's because of a timing issue of the 555. Can you think of any reason why this could be the case?

First as Jim's suggests, check power supply bypass capacitors, and a place a 0u1F between ground and Ctrl pin of the NE555s.
Next, if that has not fixed it, check the output current capacity of the power supply.

'Noise' gives baffling symptoms.

Spikes on the supply get blamed for lots of deviant behaviour and encourage paranoia. You can only be sure it is not spikes by swamping the supplies with capacitance during testing. To identify supply problems, monitor the power supply voltage while testing the circuit at high duty-cycle.

 

[jim hardy]

if you add parallel capacitance to the inductive motor then you need another inductor between the motor and the the PWM drive transistor.

right, i missed that.
Eliminating noise "at the source" wants capacitance right across the motor terminals
keeping capacitor charging current out of the PWM driver wants its low side to circuit common

so compromise by either
a small-ish cap that won't hurt Mr TIP122 placed right across motor terminals, flywheel diode there too
or
return capacitor's low side to ckt common physically close to Mr TIP122's emitter
or
as you suggested a di/dt inductor for Mr TIP122, also wrapped by flywheel diode .

, guys !

 

[enJayneering]

Wow, great response time and advice. I can't thank you guys enough!

I have already placed a bypass cap between vcc and ground (which I thought was to reduce spikes) and have a flyback diode in parallel with the motor as well.
Should I switch the bypass cap from vcc to ctrl, or should I just add another capacitor to the ctrl pin?

I will add a capacitor to my dc motor (maybe something in the pico range?) and will double check the rating for the diode I'm currently using.

 

[Baluncore]

Should I switch the bypass cap from vcc to ctrl, or should I just add another capacitor to the ctrl pin?

You need both.
I would put an 0u1F ceramic cap between Vcc and ground as close as possible to each NE555, pins 1 and 8. That will handle the high frequency noise. Add one 10uF electrolytic, preferably 100uF, between Vcc and ground on each PCB or module. That will handle lower frequency noise.
Prototyping boards are notorious for encouraging HF noise.

Note that when ceramic 0u1F “skycaps” fail they become leaky. When used in power supplies the leaky layers are fused, but for high impedance signals there is insufficient current available to clear the leakage. So add a low leakage polyester 0u1F or 10nF between ground and the Ctrl pin of each NE555.

As it is, motor inductance will keep the current flowing should the motor brushes bounce. If RF interference or a burnt commutator is a problem then it can be snubbed later. At this stage I would NOT put a capacitor across the motor as it will increase the current noise on the supply.

 

[enJayneering]

Okay, so I tried the suggestions you made and added in a few more capacitors. I did notice a reduction in noise on the oscilloscope but I still have the same problem at high duty cycles.

Here's a picture of the output signal of my 555 timer when the motor is running. There is a noisy spike before it reaches its max voltage:

 

[Baluncore]

Pictures of transitions don't help much. No idea what the time and voltage scales were. You did not say which NE555 output that was. That wobble on the trace is probably due to links on the prototyping board and the length of the ground lead on the oscilloscope.

You need to look at the power supply rail relative to ground with the oscilloscope.
Does it maintain the correct voltage? What are the biggest noise deviations on the supply?

You know what circuit you are testing, we do not. Can you produce a schematic for your current receiver and output driver circuit with part numbers. What motor driver transistor are you using?
Without that information we really cannot be much help at all.

 

[jim hardy]

check the output current capacity of the power supply.

Did that get done?

 

[enJayneering]

I'm using 3 AAA batteries from Duracell for my power source which should be able to supply the required current to the motor. Based on the data sheet of the motor, the maximum rated load current is 270 mA and the starting current is 1300 mA max. However, because I'm using PWM, I am not sure if the motor has to draw that starting current every time it starts its on-pulse. If this is the case than my batteries will have very little life duration. What do you guys think?

I will post a picture later today that is a clearer image of the oscilloscope of the receiver circuit along with its updated schematic.

___________________________
data sheet for motor:
https://media.digikey.com/pdf/Data%20Sheets/NMB-MAT/PPN7PA12C1.pdf

datasheet for battery:
http://professional.duracell.com/downloads//datasheets/product/Industrial/Industrial AAA.pdf

 

[enJayneering]

Here is a clearer image of my oscilloscope readings with the time scale shown:

I still have to add in the bypass capacitors that I'm using but here is the main circuit and part id's: