# Servo Circuit

Discussion in 'Sensors and Actuators' started by barnzey90, Dec 22, 2013.

1. ### barnzey90

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Dec 22, 2013
Hi Guys, first post here so ill try and be as detailed as possible as to what I am trying to do

I need to build a servo controller to control a 6V servo with a source voltage of 12V. The servo needs to move from position A to position B when a trigger voltage (12V) is applied.

So I have a constant 12V to feed the circuit, and by default would put the servo in position A. I then apply a trigger voltage to the circuit which moves the servo to position B. then if I remove the trigger voltage the servo reverts to position A. The circuit also needs to remember these positions with total power loss.

If anybody could help that would be great I'm capable of assembling the circuits, but don't have the knowledge to design them. I did a google for servo controller designs and the most popular method seems to be with a 555 timer, but these all seem to use 5V (I need to run off 12V and control a 6V servo) and just use a pot to move the servo. I thought maybe I could run two of the 555 designs in parallel with a relay so to switch between when the trigger is applied, and replace the pots with resistors of the right value but I'm not sure it would work..

TIA

2. ### jpanhalt

426
4
Nov 12, 2013
The 555 will run off of up to 18V. The bigger problem will be your servo. If it is limited to a maximum of 6V (some are rated a bit higher to accommodate lithium batteries), you will need a voltage regulator.

That may not be a major issue, and a typical, adjustable-voltage, linear regulator may suffice (e.g., an LM317). But, you need to consider the heat dissipated by that type of regulator. Dropping from 12V to 6V means the regulator will need to dissipate a wattage equal to 6 times the current used. With a small servo that draws only 100 mA, then only 0.6W is dissipated. The regulator will get warm, but still be OK. If the servo needs 1A, then 6W is needed, and you will need a good heatsink or different type of regulator.

What is the current draw of the servo you will be using? Is it a typical model RC servo? Can you give us the brand and model number? Is it a "digital" servo?

As for generating the two positions, that can be done with a 555 (or Schmitt inverter) and a switch or with a microcontroller. Which do you prefer? If it is a digital servo, some do not work particularly well with the most simple 555 circuits.

John

3. ### kcoer

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Dec 20, 2013
barnzey90 So once the servo moves from A to B you will need it to go back to position A right? You might have to add a DPDT momentary switch, relay, or H bridge depending on what kind of trigger this is. Is the switch a transistor or a momentary switch?

Last edited: Dec 22, 2013

5,164
1,087
Dec 18, 2013
Is this an RC servo? If this is the case then most run on 4.8V to 6V so you need to drive it with PWM. 1 to 2ms with 20ms space between pulses is what's normally used.

5. ### barnzey90

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Dec 22, 2013
Servo I had planned to use is this one: http://www.jaycar.com.au/productView.asp?ID=YM2763 But if there is one that would be more suitable then I'll buy them.

jpanhalt: dropping the voltage to 6V is not a problem, so lets assume that the source voltage and trigger voltage are now 6V. Also I would prefer to use a 555 and a switch as I'm more familiar with it but at the same time it needs to be reliable so if there is something better to use I'm open to it.

kcoer: Yes it needs to go back to Position A. The trigger is just a voltage (12v or 6V, whichever is easier) so could use a relay to mimic a switch.

6. ### jpanhalt

426
4
Nov 12, 2013
Here are two links for 555-based controllers.

This version uses two 555's and may gove a little finer control:

This uses a single 555:
http://sarconastic.tripod.com/servodriver.html

There are many versions of single-555 drivers. For more examples, just search on Google. The trick is to get a duty cycle of <<50% with the 555. That is the purpose of the diode(s). They steer the charging and discharging currents through different paths. The problem I have seen with single-555 drivers is at very low duty cycles, you may get instability. [Edited] If that happens, increase frequency to 60 to 80 Hz (some servos work perfectly well to 100 Hz or more). Then the duty cycle for the same position will be a greater percentage. Note: Servo position depends on the absolute width of the pulse, not its percentage of the cycle (i.e., not on duty cycle). Nevertheless, people still refer to it as duty cycle.

I am assuming you do not have an oscilloscope. If you did, setting the positions and duty cycles is a snap. Without a scope or a DMM that gives duty cycle and frequency, it can be a bit of trial and error. Be sure to download the datasheet and an application note for the 555. They will have formulas for calculating frequency and duty cycle.

I would set up one position with resistors, then put a larger resistor in parallel with the pulse width resistor and switch it in or out to give the second position. You could also use a DPDT switch to accomplish the same thing. The fist option would allow you to use a simple SPST pushbutton switch , which may suit your needs better.

More than 10 years ago, most of the circuits were 555 based. Today, most of the circuits you find will be microcontroller based, as they are much easier to set end points for.

At this point, pick the circuit you plan to make and post it or a link here. Then, we can help select resistor sizes and be more specific about the switch.

John

Edit comment: The sentence marker "edited" originally read, "reduce the frequency from 50 Hz to 30 or 40 Hz." That obviously doesn't make sense in the context of the statement. The sentence now reads as intended. Sorry for the momentary brain lapse.

Last edited: Dec 23, 2013