Circuit Analysis of IR transmitter/receiver

[Baluncore]

I'm not sure how you have modified the circuit.
Have you installed the IR receiver?
What mosfet do you use?
Does the IR receiver have sufficient output current and voltage change to drive that mosfet?

 

[enJayneering]

1) Yes, here is a digital version of the breadboard I have wired up:

2) The MOSFET that I am using is a N-Channel transistor MOSFET IRF510 from RadioShack.
3) I'm not sure about the current, but I am getting around 4 volts from the transistor output as a square wave.

I have probed between the drain pin and ground and have gotten this square wave. However, over time the amplitude of the signal seems to level out.

 

[Baluncore]

@enJayneering.

1. You show a 10k resistor in series with the motor. That would explain the inactivity.

2. You show a TIP120 darlington, but claim to use IRF510 MOSFET. Which is it?

3. You show a 220 ohm resistor in series with the output of the IR receiver module. That current limiter would only be needed with a bipolar TIP120, not with a MOSFET.

4. The IRF510 MOSFET will only conduct about 100 mA with 4 volts on the gate. It has insufficient drive voltage.

 

[enJayneering]

I am using a IRF510 MOSFET, but the program I was using only had the TIP120.

Also, based on what you said, I made some modifications to the circuit, and I removed the unnecessary current limiting resistors. This didn't solve the problem but I'm sure there is a simple principle that I'm over looking

Right now, I just want to get the motor drive working before I work on the flip flop of the signal. The schematic that I'm currently using is exactly the same as the previous one minus the MOSFET and the resistor. The readings from my oscilloscope show that I have a 15 Hz square waves with an amplitude of around 5 volts. I'm not too sure about the current, because my multimeter measurements increase over time after I hook it up to the circuit (Using both AC and DC settings). Based on the data sheet for my motor the Rated Load Current is 270 mA max (https://media.digikey.com/pdf/Data%20Sheets/NMB-MAT/PPN7PA12C1.pdf)

The questions I have are:
1) How should I probe to get accurate readings of the current when it's a square wave?
2) Is the lack of current the only reason this circuit fails?
3) What current spec should I be looking at on the data sheet to determine the necessary current?

Sorry for the many questions. I took a circuit analysis class last semester but they never talked about working with square waves and designing off of data sheets. Let me know if there is any good readings for this type of circuit I am trying to build

Thanks!

 

[Baluncore]

A digital multimeter is no good for measuring a square wave, voltage or current. It gets confused by beats between the sampling and the changing signal, so tried to change range.

You need to drive the mosfet with a low impedance driver capable of about 200 mA so as to switch the gate quickly and reduce power dissipation.

You need to use a MOSFET with a logic level input. It must switch from full off to full on in less than the voltage swing available.

 

[enJayneering]

So I looked into it, and it seems like current probes are very expensive. However, based on some data sheets of various IR receivers, it looks like the output current of the sensor is around 5 mA at a max. In which case I can see why my motor is not moving.

What would the solution be to increase the current from this output?

 

[Baluncore]

What would the solution be to increase the current from this output?

Originally, the NE555 was there as a pulse extender and a 200 mA driver. Now the 555 has gone, (because it was inverting the signal?), you will need to replace it, or use a MOSFET driver chip or a complementary pair of emitter followers as shown here. https://en.wikipedia.org/wiki/Push–pull_output
The important thing is to maintain the gate drive voltage and find a logic level MOSFET.

You can monitor the current with the oscilloscope by introducing an 0.1 ohm resistor into the power supply ground lead and using ohms law.

 

[enJayneering]

That's a clever way to get the current. Thanks!

I only took out the 555 timer to simplify the circuit for myself so I could better understand each individual component. I didn't realize the effect it would have on the current to the transistor. Should the drain current of the MOSFET be above the value that the motor requires?

I'm going to order in a new MOSFET and will keep testing to see if the threshold voltage was the problem.
Would this work? http://au.element14.com/infineon/bss214nwh6327xtsa1/mosfet-aec-q101-n-ch-20v-sot-323/dp/2709852

 

[Baluncore]

Only 20V and 1.5 amp is a little delicate for a beginners prototype motor driver, but it has excellent gate threshold characteristics. It has low gate capacitance which is good. See graphs 5 and 7 on page 5 of the data sheet available from Element14.
Have a look in the selection tables for something with similarly suitable Vgd characteristics but with 10 amps and 35 volts or better. It needs to survive your experimentation, so specify a heavy TO-220 package rather than a surface mount.

When the NE555 is used as an inverting buffer it needs no additional components, tie pins 2 and 6 as the input. Take the output from pin 3. Tie pins 4 and 8 to Vcc=+5V. Pin 1 to 0V=ground.

 

[Baluncore]

Should the drain current of the MOSFET be above the value that the motor requires?

Yes, for testing, preferably above the stalled motor current. = V_supply / R_motor.

 

[enJayneering]

Thanks for the advice. I was thinking that my motor was small enough to get away with 1.5 amps, however I see that it's much better to go beyond what is required while prototyping. I have ordered in some new parts based on your recommendations.

Because I don't have the parts yet to build the receiver based on your schematic, I decided to test out the original schematic that I first posted on this forum. It works and manages to drive the motor but I remember you saying that 555 timer would miss some of the pulses and thus decrease the drive to the motor. Is this the only reason to change the configuration of the 555 timer?

Also, you've suggested a few configurations for the 555 timer that I hadn't seen before. How do you know which way to wire it up? Does that come from a better understanding of the comparators and internal workings, or does it come with experience of working with them a lot?

 

[Baluncore]

Also, you've suggested a few configurations for the 555 timer that I hadn't seen before. How do you know which way to wire it up? Does that come from a better understanding of the comparators and internal workings, or does it come with experience of working with them a lot?

The NE555 did not become a flexible commodity until I realized that it had two comparators driving an RS flip-flop.

It was always a bit mysterious because the NE555 was called a timer and described as an astable oscillator in the original data sheet and application notes. The simplest application, needing no external components, is an inverting schmitt-trigger with 200 mA output drive and input thresholds of 1/3 and 2/3 Vcc. Unfortunately that was not shown in the original data sheet. The NE555 also has an inconvenient pin-out that often needs connection of pin 4 to 8 and pin 2 to 6. That made the layout non-obvious because those tracks needed to cross diagonally under the chip.

 

[jim hardy]

The simplest application, needing no external components, is an inverting schmitt-trigger with 200 mA output drive and input thresholds of 1/3 and 2/3 Vcc.

I love 'em. They'll interface almost anything.

 

[enJayneering]

Yeah, I watched a few videos on the internal workings and learned how the comparators create the flip flop but did not dive any deeper than that. I didn't realize all the different ways to drive the 555.

I've managed to set up the rest of the receiver circuit and I have some good news! I am able to drive the motor at different speeds based on the pwm of my transmitter circuit. I am running into one issue though. 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?

Thanks

 

[jim hardy]

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?

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...

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.

Spikes on Vcc get into the 555's internal voltage divider and can cause false operation.. Some capacitance from CTRL to common should fix that . It's one of those little precautions buried in the datasheet's fine print.

'Noise' gives baffling symptoms. Above might help keep you from chasing it for days on end(as i have so often done).

Good luck guys - interesting thread !

 

[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: