Hello,
I have a 6V dc motor which I would like to get to run for 24 seconds and then to stop for 4 seconds and then to start again (and again).

The following is the pwm circuit that makes the motor run non-stop:



This first circuit works fine.

To make the motor start/stop I am using a second astable circuit:


taken from: https://www.electronics-tutorials.ws/waveforms/555_oscillator.html

I have calculated the C, R1 and R2 and this circuit also works fine with a LED. (It stays on for 24 seconds and off for 4 seconds).

Now when I try to combine the two circuits things get tricky.

I tried connecting the output of the second astable circuit (picture number 2) to pin 5 (control) and to pin 4 (reset) of the 555 from the first astable circuit.
(picture number 1)
I tried connecting the output to pin 8 (as the source).
I tried using a transistor in a forward emitter configuration on the output of the second astable circuit to amplify the signal back to the 6 volts I am using as a source..
I tried using a transistor as a switch, because I wanted to keep the low level at a low (so it wouldn't get amplified too much)

Whatever I tried the result is always the same. The circuit works fine with LEDS but not with a motor.
With a motor the LED that I kept as a reference starts blinking around 10 seconds and the 24 second stop never happens (and the motor doesn't stop either, of course)

Today I have tried a new approach. I combined the two outputs (from the two 555's) using a cd 4081 IC (an AND gate), and then I applied the new signal to the base of Q1 from circuit 1 (picture 1). Again the motor never stops and the reference LED starts to blink in about 10 seconds.
Becaus

e this happens with a delay I guessed it must be coming from the capacitor that is in the second circuit. I use a 100micro Farad capacitor.
I thought that maybe the motor draws too much current and the capacitor saturates in the first 10 seconds, so I tried putting a higher capacitor and using smaller resistors (to keep the charge/discharge times the same). The LED starts to blink even sooner than 10 seconds. I also tried a smaller capacitor.

The only things I haven't tried is using pull-up and pull-down resistors since not all the components are from the same logic families. Could this be the problem?

So my question can be summed up to the following: Why does a circuit that works fine with LEDs stop working (and start oscillating randomly) when at the output the load is a motor that draws a much larger current?
Thanks

 

Hi

Pull down the reset pin of the PWM timer with a 10k resistor.
Then connect the output pin of the cycle timer to the reset pin of the PWM timer.

BTW-
The RC values I used:
R1=460K, R2=86K, C=68uF = ~0.035 MHz, On=24s, Off=4s

eT

 

Quite likely radiated energy from the motor getting into the control circuit (place a .01 uf capacitor from each motor terminal to case as a starter in fixing this if it is the problem.

Even more likely is conducted interference from the motor circuit through the power supply or teh power supply goinog out of regulation. Assure your power supply is adequate and if necessary isolate the motor circuit from the battery with a filter and/or use a low pass RC filter to quiet the VCC in the NE555 circuit.

And always make sure your grounds are tidy and current to the motor does not pass through the timer grounds.

 

Connect pin 3 of the Astable to pin 4 of the pwm...this will turn the pwm on/off...

 

Connect pin 3 of the Astable to pin 4 of the of the pwm...this will turn the pwm on/off...

Here's a LTspice simulation of that configuration showing the left 555 inhibiting the operation of the right 555.
(The 555's have different time periods than wanted, but it demonstrates the principle).

As Dick C noted, keep the motor current separate from the 555 circuit.
Run separate wires from the Q1 emitter and the motor plus to the power supply, to minimize the effect of the motor current on your circuit.
Also good supply filtering with capacitors at the circuit and the motor is a must.

 

Here's a LTspice simulation of that configuration showing the left 555 inhibiting the operation of the right 555.
(The 555's have different time periods than wanted, but it demonstrates the principle).

As Dick C noted, keep the motor current separate from the 555 circuit.
Run separate wires from the Q1 emitter and the motor plus to the power supply, to minimize the effect of the motor current on your circuit.
Also good supply filtering with capacitors at the circuit and the motor is a must.

View attachment 178342

You should pull U3 pin 4 to ground with a 10k resistor to ensure the chip resets.

eT

 

You should pull U3 pin 4 to ground with a 10k resistor to ensure the chip resets.

That makes no sense to me.
Reset when?
If you mean at startup, U3 will start fine without an initial reset.
Most 555 circuits never even use the Reset input and it's permanently tied high.

And since the maximum 555 reset current is 0.4mA and the minimum reset voltage is 0.4V, any reset resistor would need to be no larger than 1kΩ.
10kΩ won't work for that purpose.

 

Why would you need that when it is connected to the output of the first 555?

Bob

 

That makes no sense to me.
Reset when?

Whenever the output of U2 is low.

If you mean at startup, U3 will start fine without an initial reset.

Not what I mean.

Most 555 circuits never even use the Reset input and it's permanently tied high.

Yup.

And since the maximum 555 reset current is 0.4mA and the minimum reset voltage is 0.4V, any reset resistor would need to be no larger than 1kΩ.
10kΩ won't work for that purpose.

Ok...so use 1k if you like

The point is there are conditions where the reset pin can function out of spec.
The cost of a resistor is trivial but the extra reliability is worth it. Especially if making PCB's.

But...lets leave it up to the TS....

eT

 

Whenever the output of U2 is low.

I assume you mean U1.

The point is there are conditions where the reset pin can function out of spec.

Of course, that's true of just about any signal.
But why do you think that occurs here?
The 555 output maximum low is <0.4V with a small load, so is more than sufficient to Reset U3 without any added pulldown resistor.
You might try reading the spec sheet.

 

I assume you mean U1.

Of course, that's true of just about any signal.
But why do you think that occurs here?

Not possible to predict. The pulldown resistor is a preventative measure.

The 555 output maximum low is <0.4V with a small load, so is more than sufficient to Reset U3 without any added pulldown resistor.
You might try reading the spec sheet.

I have...many times...

No one is forcing you to pull down the reset. Do what you want. Its a suggestion for the TS to consider.

eT

 

@eetech00

So do you put a pull down resistor on every input that drive with a totem pole output?

Bob

 

@eetech00

So do you put a pull down resistor on every input that drive with a totem pole output?

Bob

I do on a NE555...

 

Not possible to predict. The pulldown resistor is a preventative measure.

It's entirely possible to predict.
You look at the spec sheet input voltage and requirements for the Reset function, and the output voltage and current characteristics of the Out terminal.
If the output characteristics are more than adequate to meed the Reset input requirements, then I can predict, with a great deal of confidence, that the Reset function will work reliably.
A pulldown resistor here is waste of money, and circuit area.

I have...many times...

Then perhaps a more careful reading.

 

It's entirely possible to predict.
You look at the spec sheet input voltage and requirements for the Reset function, and the output voltage and current characteristics of the Out terminal.
If the output characteristics are more than adequate to meed the Reset input requirements, then I can predict, with a great deal of confidence, that the Reset function will work reliably.
A pulldown resistor here is waste of money, and circuit area.
Then perhaps a more careful reading.

Your certainly entitled to your opinion....

eT

 

Ok, everyone
Thank you for your help.
I put a 10k pull-down like eetech00 said and now the motor works much better.
Usually pin 4 and 8 get connected to Vcc Though I had disconnected 4 from 8 when I connected to RESET, I would have never thought that connecting 4 to ground would solve the problem, but I do see the logic behind it.
The strange thing is that now the on/off time of the motor is not so precise anymore. I will try everything Mr Cappels and others have suggested I do. Thanks.

 

Not so precise as it was for the LED I mean. Because before the motor didn't stop at all.

 

Your certainly entitled to your opinion....

But not our own facts.

 

Sorry crutschow, but I am with eetech00. Here in the real world nothing is perfect and I know that the waste of that specific 1/4 cent resistor can save a lot of aggravation. I usually respect your knowledge, but shake your head man your eyes are stuck. Your Curmudgeon tag safe without the hissy fit.

 

I think that would be a wasted 1/4 cent resistor. Take a look at the simplified schematic of the NE55 and tell me how a pull-down resistor could make a difference.Every unnecessary component is a chance for a problem. If you are just making a one-off for your own experience, who cares, but if you are designing for production, take crutschow's advice seriously.