I have a perenial problem I am sure someone will be able to help with. A lot of my student's gcse projects (I am a technology teacher) use 555 monostables controlling cheap, noisy, (electrically) motors through a tip 121.This leads to interference with the time period, and if an astable is controlled by the mono as well, the astable becomes totally useless. I have tried all sorts of capacitors in all sorts of places in the circuit to try to alleviate the problem but nothing does it completely. There must be a PROPER way to sort this out without having to resort to more expensive motors. Any help gratefully received
Thanks
Tony at Rawmarsh

 

I still don't know what exactly you have in mind. If I get it right, you want to use one 555 as monostable to trigger second 555 (which probably serves as PWM speed regulator). This drives TIP121 and small DC motor.
Two 555 chips are not coupled correctly and somehow you get lots of interference with ???
I'm not sure what interferes with what. Probably you forgot to install
diode across motor terminals. If the interfernce is between two 555 chips, you can try to run everything without motor. See if circuit starts behaving normally.

 

can you post a block diagram or describe how this circuit is constructed?

 

Avoid placing capacitors haphazardly throughout. Make sure the motors have "freewheel" diodes as well as a zener for voltage spike suppresion. A capacitor here will help as well. Try 0.01 uF ceramic type. Place all of these components as close to the motor as possible. These parts all help to suppress voltage noise from getting conducted back into your circuits.

The second thing you should also do is place 0.01 uF ceramic type bypass capacitors right at the power supplies pins of the 555's and any other IC you have in the circuit. These will help with any noise that is coupled onto the power lines from affecting your outputs.

If you suspect electromagnetic interference (which is probable only in the most extreme cases), try and get as much distance from your control circuits to your motor as possible.

Another thing to examine is the current return path from the motor. This is probably your ground for the whole system(or is the motor isolated?)
If you do not have an isolated design, make sure you do not run this "high current" ground through the grounds of your 555 grounds. There is inductance in the ground path and high current pulses will create voltage spikes that your 555's and timer circuits will see and that will affect there timings. If you suspect a ground problem, make a "star" ground system by grouping all of the grounds of your timing circuits together and run that wire back to the power supply. Then take the ground of your motor and run a _seperate_ wire for that back to your power supply. In essence, all of your grounds connect at one point only and that connection point is AT the power supply.

maybe these things will help. I am taking stabs in the dark here since we really need to see your system to be of more help.

 

First thanks for the replies, I have posted a block diagram to explain exactly what I am trying to do. However I have no diodes in my circuit at the moment and this appears to be important. If I disconnect the motor the rest of the circuit works fine, so it definately a motor interference problem. Which diodes do I need and exactly where do they go, straight across the motor terminals? Thanks in anticipation of any advice
Tony

Attached Images

motor_circuit.jpg (65.5 KB, 250 views)

 

Yes please put the diode in. You can try 1N4001 or similar. I suspect that you may have your motor ground connection interfering with the ground of your 555 (or VCC) Can you put an oscilloscope and check the power & ground fidelity? You might want to try a star ground arrangement if you find that this is where the interference is getting into your timing circuits.
But try the diode & zener first (since it will be very easy)

Could you revise your diagram a bit and show how the power supplies & ground is laid-out? I mean try and draw lines just like you have the wires running. The closer the representation to your real circuit, the better.

 

Here is the circuit diagram as it has been built, as you can see, no diodes, and the negative of the motor going back into the negative of the ICs. Where exactly does the zener and the diode go?

Attached Images

circuit_for_motor.JPG (43.4 KB, 227 views)