DC Motor / Circuit.

Thread Starter


Joined Mar 26, 2009
Ok in my circuit i simply have a 555 timer which makes an LED blink on and off I have a regulated input voltage (from an LM137) at 6volts coming into the 555. now two questions 1. why? 2. is there a way to prevent the following.

Between the Circuit (on the breadboard) and the output from the LM317, I placed a DC motor i pulled out from an old cd player, now the 555 goes weird I measure the voltage and it sticks at 6v according to the voltage meter but the LED flashing rate has gone from 350ms (1/4 or so a second flash rate) to about 10ms constantly flashing...

I tried placing a cap 440uf across -/+ of the IC i even wired up my own large inductance coil to try and filter out the voltage to see if that's causing the problem obviously there's a voltage ripple from the DC motor but the meter says it does not as in a constant 6v before/after placing the motor on, i don't need to fix this I'm curious as to why this happens and what would be needed to stop it....


Joined Mar 6, 2009
Your meter would probably not register any noise introduced by the DC motor. If the motor is a simple commutator type, then it's not unexpected there is some interference. Have you connected all the '555' input pins to their typical levels - particularly 4 & 5.

You could try decoupling the '555' supply point with a resistor & capacitor of some reasonable value (how much current does it draw when the LED's are functioning normally?). Try 1K and 100uF. Also use a (1uF?) ceramic bypass in parallel with the 100uF. You would connect the 6V supply to the resistor on one end and the other end of resistor to the '555' power pin 8 (and enable pin 4). Bypass the power pin to ground with the suggested capacitors.

Of course the problem could be with the LM317 reg. getting upset. Have you bypassed it's inputs and outputs ???

Not much else I can suggest.



Joined Feb 28, 2009
Try putting a diode in series with the motor lead (polarity sensitive so try it both directions) NE555 timers are sensitive to the inductive kick from any inductive load and can cause latch-up. You could also try isolating the motor with a single transistor driver. Also, make sure the motor is not trying to draw more current than the 555 can handle..


Joined Feb 19, 2009
An oscilloscope between 555 ground and V+ would answer the question.

A 0.1uF decoupling cap across the 555 V+ and gnd, and add a diode across the motor, stripe pointing towards positive.

Might also add a 0.1uF cap across the motor terminals to minimize RF as well. You can try them one at a time:

A) Kickback diode across motor
B) Decoupling cap across 555, recommend this anyway, even if diode 'fixes' issue
C) noise suppression cap across motor terminals.

Use ceramic 0.1uF caps for both cap applications.