555 timer with servo - 12v ?

[alpha85]

Hi guys. I have not produced a circuit for a few years now (used to be producing a few a week!).

I need a servo to turn 130 degress then pause (standard PWM). But don't want it using RC or any controllers etc, just want it so that when power is applied....the servo moves.

A 555 circuit wth potetiometer (so I can tune the angle!) should be ok for this but I have a couple of problems.

1. I will be using 12v from a motorbike power supply.

2. I need the servo to return to the 'zero' or start position when power is removed. Capacitor to retain anough charge to allow this? Just a plain signal, no PWM so servo should reset?

3. What is the most efficient circuit to use?

Everything will be bonded into one box with only 2 power wires (12v DC), potentiometer and servo arm extending.

What would be the ideal voltage for this circuit? I think the servo should run at 6v.

Any help appreciated guys

 

[alpha85]

Any help at all?...

 

[colin55]

Here are the circuits you need:
**broken link removed**
**broken link removed**
A pot can be used to control the position of the servo by using the circuit above. It produces a positive pulse between about 0.9 milliseconds and 2.1 milliseconds. The off period between pulses is about 40 milliseconds. This can be shortened by reducing the value of the 3M3 resistor.

 

[alpha85]

Ok fantastic. What about when I remove the power. Need the servo to go back to its starting position. Have a capictor hold a charge, enough to reset the servo to zero? There will be no external power available....so maybe use the 12v to switch a twin relay that connects this circuit, then when power is removed the relay unlatches - a circuit with a charged capacitor is connected to the servo (no PWM so will reset it to zero) ?

 

[alpha85]

This will all be running directly from a motorcycle battery so need to reduce the 12v down to 6v for the entire circuit. And will prob need a fuse and some way of ensuring this cannot interfere with ignition and the other items connected to the battery?

Think it is 12v, 8ah

 

[colin55]

The circuits I posted are just to get you started. You need to know the voltage of the servo. It is generally 4.5v to 6v. You can charge 1 or 2 3.6v lithium cells in series via a resistor and use the energy in the cell(s) to drive the servo. A capacitor (electrolytic) is generally not suitable.

 

[Sceadwian]

A large electrolytic capacitor on the 12V side before a 5 volt regulator may provide enough hold up time to reset the servo position, experiment with the largest electrolytic you have, but keep in mind servo's have no holding torque without power so the control arm will float with any force applied to the output shaft. If the load on the output shaft of the servo is light enough a cap may work, again though it has to be on the 12V side of the regulator, as soon as the power is lost the voltage will quickly drop but the time it takes to drop from 12 to 5 volts when the regulator drops out could be enough. One possible problem though is as soon as the voltage drops bellow 5V the applied VCC to the 555 will change so this make cause the timer circuit to send a distended PWM to the servo, all it takes is one bad pulse to tweak a servo a lot.

 

[alpha85]

Thanks guys...all of this is really helping!

The circuit will be for a flip plate when I take my motorcycle on track days. So when i'm on the road and it is off, the plate would have to remain in the zero or off positions with up to 120mph wind against the plate. Guessing even a 15kg torque servo would move if not secured with a spring?

So might have to incoporate a spring and use this to ensure the servo always resets when power is removed.

Will use the 555 and potentiometer to determine the angle the plate flips to when I take it on the track (people record plate details to come and 'borrow' your bike when you are at work).

So all I need to worry about is converting the 12v / 8ah down the 555 / servo voltage and ensuring no feedback or risk tot eh bike electricals.

 

[alpha85]

This is what I am trying to acheive (or using as inspiration). This kit has many flaws though :

https://www.flipaplate.com/GoldKit.html

1. It does not control the servo angle, it simply uses full turn until the plate contacts the bike (think it actually uses a 12v motor :-o )
2. The build quality is poor (mine will be milled and powder coated aluminium)
3. The parts are very cheap and poor quality (look at the switch for example)
4. Expensive - I can build one cheaper and higher quality

I will also be using a simpler spring mechanism. Connecting the servo control arm to the plate, and a spring in the opposite direction - to correct when power removed. This will ensure not large collar springs are required and keep the design to a minimum.

Once the circuit is PCB'd it will be attached to the side of the servo and the entire unit then surrounded with a cardboard frame and filled with epoxy (then frame removed) creating one PCB / servo unit.

 

[alpha85]

Hi guys

I have been looking round some more. I think the best option would be to use a 555 to lift the plate (by controlling the servo). I would like a joule thief and capacitor to then provide a flat signal to reset the servo when the main 12v power supply is swiched off. Seperate the two circuits (PWM from 55 / flat + capacitor as power) using a double relay.

Thoughts?

If somebody could help me out with a circuit diagram I would be very grateful. From the 12v / 8ah through to the servo + the joule thief capacitor setup to reset the servo when power removed?

 

[Sceadwian]

Alpha, as I stated, start with the basics first, get a 555 circuit that can output a servo signal and add from there, there are plenty of schematics for 555 servo signal generators colins will work fine for getting you the basic servo signal and you can modify / proceed from there, you can't jump in head first and expect to swim gotta learn each step as you go.