Servo Control Problem

Hi, Newbie question. I have made a servo control circuit and it does not work. It will be very nice if someone in this NG can help. -------------------------------------------------

SERVO: Sanwa, SRM 102, 6 volts, 3.3 Kg-cm torque;

Leads are black,red and blue. I am assuming them to be ground, +ve, signal. If I switch on the power with out connecting the signal wire, (a) it moves to the left if it is not already there. (b) If it is already there, then there is no movement.

If I connect the signal there is no change in the behaviour of the servo. It is like in a, b above.

--------------------------------------------------- Circuit: It is by tony von roon

Even though I could access this page yesterday, today it is giving "404 not found" error)

Anyway the circuit schematic is very unlikely to be wrong, so the problem ought to be in my implementation...

It is a circuit based on the 555 chip, to generate 1ms to 2 ms pulses. The output is routed via a transistor. I have put a LED on output ( pin no 3 ) of the 555 chip and it glows when i switch on the circuit.

I dont have a oscilloscope nor do i have a frequency counter.

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Any ideas about what could be wrong ? How do i find the pulse width ? .... Please note that I am a beginner in electronics. Thanks in advance. Venks

Hard to recommend an approach without knowing what you can get your hands on. An obvious solution would be a gated counter, either a dedicated counter chip or a counter/timer inside a microcontroller.

hook a loudspeaker to pin3 (and ground) through 220 ohm resistor (you should hear a buzz and the nature of the buzz should change as you adjust the control)

as a rough indication the LED's apparent brightness will change.

If you move the led a known fast speed the past a fixed scale the pulses of light will appear to stretch... or you could use capture it using a computers audio line in and view it on a "wave editor" (eg: audacity)

but it's probably easier to replace the capacitor with one with

if you don't have a resistor in series with the LED add one (anything between 470 ohms and 2K should be fine there.)

Bye. Jasen

The servo should not move at all if you only connect the +ve and -ve. You may have a faulty servo.

The length of the pulse determines the position of the servo - approx 0.8mS at one extreme and around 2.2mS at the other. 1.5mS should be approx. middle. Each pulse should be separated with a low of approx 20mS. Although longer than this just means that the servo takes longer to reach its position.

If you are connecting all three wires, then it looks like you are getting pulses out from the circuit (is it similar to this one?

If it is similar, you may want to play around with the value of R2.

Tim