- Jan 1, 1970
I'm building a little 555 timer circuit that turns a small geared 3VDC
motor on for an as yet undetermined time (estimated max 15seconds).
When calculating the RC for the timing is it best to keep the
capacitor at or under 1uF with larger resistor values so as to avoid
using electrolytics? I thought they were leaky with wide tolerances
and not good for timing applications.
Not necessarily. You also have to consider the bias current
requirements of the 555. If the capacitor charging current approaches
the bias current, the timing accuracy will also be degraded. If you
adjust the time with a variable resistor, electrolytic caps can be
pretty good for a long time. What timing accuracy are you needing?
There is also a timing trick for electrolytics that improves the
leakage situation for long times. Stack to wo similar capacitors in
series across the supply, and use the center point as your timing
node. This method passes the average leakage straight through and
only the difference in leakage current shows up at the timer.
Most of the suitable monostable circuits I have seen so far have an
NPN transistor with resistor on the 555 output controlling a relay
with its protection diode. Is their a particular reason why a power
transistor, eg BD139, can't be used to switch the motor and eliminate
No. The relay just has very low DC drop, compared to a transistor.
If you can spare a few tenths of a volt, the transistor is fine. Keep
in mind the current capability of the 555 output and the saturation
current gain of the power transistor.
Also are there any special circuit requirements when using a motor
apart from perhaps a 0.01 - 0.1uF suppression cap across the motors
terminals? A reverse voltage diode placed somewhere?
The capacitor may help with radio frequency interference, but for good
circuit operation, a bypass capacitor across the supply right at the
points where the motor current leaves the area of the power switch is
more important. That would be from the emitter of the power switch to
the positive supply point where the motor connects. You also need a
diode from collector to positive supply rail to carry the motor's
inductive current when the switch first turns off, cathode band to the
At the moment the V+ on my proto-board is regulated 5V and I'm using
three diodes to drop the voltage for the 3V motor but eventually it'll
be battery powered with the motor being activated once or twice a day
for the max 15secs. The battery will possibly be a PP3 9V type. What
is the simplest and hopefully most efficient way of reducing a much
higher voltage for the motor? Dropping resistor, LM317 or... mmmm... a
zener regulator of some sort?
Simplest and most efficient are two different solutions. A series
resistor that sets the speed to about what you need is probably the
simplest. The most efficient would be a something like a second 555
timer set to mid ot high kilohertz frequency , gated on by the slow
555 described, above, to act as a pulse duty modulator to lower the
average voltage applied to the motor. If it puts out a pulse width
that is about 1/3 of the total cycle time, the average voltage to the
motor will be about 1/3 of the supply voltage, with no intentional
losses. And the battery current will average a little more than 1/3
of the motor current, extending the battery life, considerably. But
you will have ot pay more attention to transistor turn on and turn off
times and use a fast diode (Schottky) across the motor to keep the
switching losses low.