555 timer output issue

[Yuval Goldfracht]

Hi,

I'm trying to use the output of a 555 timer in mono-stable (one-shot) mode to control the rotation of a servo motor for a period of about 2 seconds. I'm using the RC period of the one-shot circuit to power a servo tester which will generate the pulses required for the servo. Unfortunately the circuit seems to work fine when connecting a LED to the output and trying to light it for 2 seconds but when connecting the servo tester there seems to be a problem - the tester flashes as if the voltage is inconsistent and the tester cannot perform. Attached is the scheme of the circuit with the required output function (which works fine when using a LED), the inverter an Differentiator are used to detect the rising edge of the input signal so the led/motor will work for a RC period starting from the rising edge.



Any ideas whats wrong ? How to solve ?

Thanks
Yuval

 

[BradtheRad]

R1 should be connected to the positive supply. You have it connected to 0V gnd.

The purpose is to pull the 555 trigger pin low momentarily, to start a timing cycle.

I don't know if that will solve the problem you mention.

 

[Yuval Goldfracht]

Thanks for the quick response. I understand your suggestion and tried simulating the following circuit adding a Diode and 2 inverters to reduce noise :


the results in the simulation looks great but after assembling it doesn't work. I think it has something to do with the VL of the inverters.
Any ideas how to solve this issue or am I missing something else ?

Thanks!

- - - Updated - - -

By the way I also thought about using the following circuit as a positive edge detector


I can control the pulse width by adding buffers for larger delay. do you this can work ?

Thanks

 

[BradtheRad]

Since your ciucuit works okay with a light load, I wonder about the supply voltage when the servo is attached. Does the servo draw a lot of current? Does the supply V go down when the servo operates? If so, then that is no doubt causing erratic performance.

As for triggering the 555, here is an easy method.



The push switch pulls the trigger low momentarily. It can be any negative signal, from a device which sinks current.

 

[Yuval Goldfracht]

I tried plugging the 555 timer in a monostable mode directly to the tester but got the same results so I think it's definitely an output voltage issue (also if connecting the LED in parallel to the servo the light decreases dramatically). I'll try and add 2 common-emitters to the output to increase the voltage and see if that works.