# Where can I find a servo that is controlled by a laser schematic with no arduino being used?

#### JeremyOkulovich

Joined Jun 12, 2016
8
Ive been trying so hard to find a schematic that does not have an arduino involved to control a servo with a laser. So far what I have is a laser pointing to a photoresistor that is the trigger for the 555 chip to create a pulse width. The pulse width was a bit too much for the servo so I tried to control it by putting in a low pass filter, but then the signal became too small (voltage wise) so I put the signal through an opamp and it is still not working right.
The breadboard and wires are a bit of a mess and would be hard to post a proper picture of it. If anyone knows where I can find a schematic of a basic idea that would be amazing. I'm trying hard to stay away from arduino because I would like to build everything from scratch so that I can try to really understand circuit design. Thank you for any help.

#### djsfantasi

Joined Apr 11, 2010
8,566
First, what do you want the servo to do when it detects the laser beam? What does it do when there is no light?

Detecting the laser beam depends on the ambient light. Personally, I’d start by measuring the photoresistor’s value with and without the laser beam.

What I’m suggesting is that you break your problem into two problems. Detecting the laser and controlling the servo.

With regard to the latter problem, do you understand how to control an RC Servo? Analog hobby servos are driven by varying the duty cycle of a 20 50Hz signal. It CAN be done with a single 555, but sometimes it depends on the particular servo you’re using. With a single 555, when you change the duty cycle to position the servo, you’re also changing the signal frequency. The off-spec frequency may not be recognized by the servo. This may be your issue.

IMHO, a dual 555 or 556 Servo Controller circuit (click on the link) is the best solution. The first 555 is used to generate the 20 50Hz signal with an astable multivibrator circuit. The second 555 is used to modify the duty cycle with a monostable multivibrator circuit. This circuit is what you would control with your photoresistor.

So you need to tell us about how you’re planning on using the laser. And do a little research on the circuit I provided.

UPDATE: Corrected mistake in signal frequency.

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#### LowQCab

Joined Nov 6, 2012
2,506
It will depend heavily on what You are trying to accomplish with your Project.

"A Laser", and, "A-Photo-Resistor" is far too vague,
and using a 555 to generate a PWM-Signal is not the best solution,
as it's very difficult to achieve a full range "0-to-5%" PWM-Signal with a 555,
and "a-Photo-Resistor" is likely to be too slow in response, and possibly not as sensitive
as a Photo-Diode, or Photo-Transistor,
which can be matched to the Light-Frequency of the Laser.
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#### JeremyOkulovich

Joined Jun 12, 2016
8
First, what do you want the servo to do when it detects the laser beam? What does it do when there is no light?

Detecting the laser beam depends on the ambient light. Personally, I’d start by measuring the photoresistor’s value with and without the laser beam.

What I’m suggesting is that you break your problem into two problems. Detecting the laser and controlling the servo.

With regard to the latter problem, do you understand how to control an RC Servo? Analog hobby servos are driven by varying the duty cycle of a 20Hz signal. It CAN be done with a single 555, but sometimes it depends on the particular servo you’re using. With a single 555, when you change the duty cycle to position the servo, you’re also changing the signal frequency. The off-spec frequency may not be recognized by the servo. This may be your issue.

IMHO, a dual 555 or 556 Servo Controller circuit (click on the link) is the best solution. The first 555 is used to generate the 20Hz signal with an astable multivibrator circuit. The second 555 is used to modify the duty cycle with a monostable multivibrator circuit. This circuit is what you would control with your photoresistor.

So you need to tell us about how you’re planning on using the laser. And do a little research on the circuit I provided.

Hey thank you for your response. So this here is some more detail about what I am trying to achieve. Im not home right now at my hobby bench but this is some detail.

So im basically using the laser as like a “trip wire” so once the laser is broken hitting the photoresistor then it creates the logic for the 555 to start creating the pulse width.

So I have the right logic coming out of the 555 chip its just not the right pulse width. So like if you were to use a potentiometer to control a servo with the 555 that is basically what I have going on except the “potentiometer” is a laser logic circuit. That works fine accept the servo will get stuck in one position because it goes too far onto one side and gets stuck. Its like if i was to put the pot all the way to one side and its drawing a ton of amps because it is stuck. So what i did was I tried to put a low pass filter to try to calm that down but now it is not giving me a sufficient signal and it is like not swiping all the way because it is seems like the gears are getting stuck. Maybe I just need a current limiting resistor on the output instead of low pass filter on the output. I think thats what im gonna try next if no one has an easy schematic here. Now that ive been asking around a low pass filter is probably not a good idea but I need to figure out how to get the pulse width to not be so much to the point where it is pulling too many amps and causing the servo to get stuck on one end to the point where it cant go back the other way.

#### JeremyOkulovich

Joined Jun 12, 2016
8
It will depend heavily on what You are trying to accomplish with your Project.

"A Laser", and, "A-Photo-Resistor" is far too vague,
and using a 555 to generate a PWM-Signal is not the best solution,
as it's very difficult to achieve a full range "0-to-5%" PWM-Signal with a 555,
and "a-Photo-Resistor" is likely to be too slow in response, and possibly not as sensitive
as a Photo-Diode, or Photo-Transistor,
which can be matched to the Light-Frequency of the Laser.
.
.
.
Hey thank you for your response. So this here is some more detail about what I am trying to achieve. Im not home right now at my hobby bench but this is some detail. Im basically just copying and pasting the same response I did to the previous comment just so that it will notify you that I responded.

So im basically using the laser as like a “trip wire” so once the laser is broken hitting the photoresistor then it creates the logic for the 555 to start creating the pulse width.

So I have the right logic coming out of the 555 chip its just not the right pulse width. So like if you were to use a potentiometer to control a servo with the 555 that is basically what I have going on except the “potentiometer” is a laser logic circuit. That works fine accept the servo will get stuck in one position because it goes too far onto one side and gets stuck. Its like if i was to put the pot all the way to one side and its drawing a ton of amps because it is stuck. So what i did was I tried to put a low pass filter to try to calm that down but now it is not giving me a sufficient signal and it is like not swiping all the way because it is seems like the gears are getting stuck. Maybe I just need a current limiting resistor on the output instead of low pass filter on the output. I think thats what im gonna try next if no one has an easy schematic here. Now that ive been asking around a low pass filter is probably not a good idea but I need to figure out how to get the pulse width to not be so much to the point where it is pulling too many amps and causing the servo to get stuck on one end to the point where it cant go back the other way.

#### LowQCab

Joined Nov 6, 2012
2,506
Unfortunately your explanation doesn't shed any light on the purpose of this project,
or why You are using a Laser,
or why You are using a Photo-Resistor,
or what the combination of the 2 is supposed to do, and why,
and what has any of this got to do with a Servo ?,
what is the Servo supposed to do ?,
what is the Manufacturer and Model-Number of the Servo ?
how are You generating a PWM-Signal for the Servo ?,
what is the Manufacturer and Part-Number of the Laser ?,
do You actually need a Laser instead of an ordinary LED for some reason ?,
what are You using for a Power-Supply ?
A schematic would be a great help, and pictures too if You can.
PDF Specification files for all the parts would be a real bonus.
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#### djsfantasi

Joined Apr 11, 2010
8,566
Hey thank you for your response. So this here is some more detail about what I am trying to achieve. Im not home right now at my hobby bench but this is some detail.

So im basically using the laser as like a “trip wire” so once the laser is broken hitting the photoresistor then it creates the logic for the 555 to start creating the pulse width.

So I have the right logic coming out of the 555 chip its just not the right pulse width. So like if you were to use a potentiometer to control a servo with the 555 that is basically what I have going on except the “potentiometer” is a laser logic circuit. That works fine accept the servo will get stuck in one position because it goes too far onto one side and gets stuck. Its like if i was to put the pot all the way to one side and its drawing a ton of amps because it is stuck. So what i did was I tried to put a low pass filter to try to calm that down but now it is not giving me a sufficient signal and it is like not swiping all the way because it is seems like the gears are getting stuck. Maybe I just need a current limiting resistor on the output instead of low pass filter on the output. I think thats what im gonna try next if no one has an easy schematic here. Now that ive been asking around a low pass filter is probably not a good idea but I need to figure out how to get the pulse width to not be so much to the point where it is pulling too many amps and causing the servo to get stuck on one end to the point where it cant go back the other way.
You need the circuit I posted. There are circuits built around a single 555 but they don’t work in all cases.

How to stop the servo from getting stuck depends on the servo. Search for an article on servo control.

A 50Hz (I mistakenly said 20Hz) carrier signal has a duration of 20ms. A pulse of 1.5ms positions the servo in the middle. Pulses of 1.0ms and 2.0ms turn the servo to the ends of travel. But as I said, it depends on the servo. Some have endpoints at 2.25ms and 0.75ms.

The dual 555 (or 556) circuit uses one 555 to trigger the second every 20ms (an astable multivibrator). The second produces the 1.0ms to 2.0ms pulse to position the servo when triggered (a mono stable multivibrator). You need to switch separate resistor values on the monostable multivibrator for each servo position you desire.

Any sort of filtering would likely destroy the waveform needed to control a hobby servo, so that’s why it probably didn’t work.

All bets are off if your servos are not analog servos. There are digital servos as well.

We could design this for you, but might require $250 to$300 per hour for our work. If there is a pre-designed circuit available, it might take a lot of searching

#### drjohsmith

Joined Dec 13, 2021
504
You need the circuit I posted. There are circuits built around a single 555 but they don’t work in all cases.

How to stop the servo from getting stuck depends on the servo. Search for an article on servo control.

A 50Hz (I mistakenly said 20Hz) carrier signal has a duration of 20ms. A pulse of 1.5ms positions the servo in the middle. Pulses of 1.0ms and 2.0ms turn the servo to the ends of travel. But as I said, it depends on the servo. Some have endpoints at 2.25ms and 0.75ms.

The dual 555 (or 556) circuit uses one 555 to trigger the second every 20ms (an astable multivibrator). The second produces the 1.0ms to 2.0ms pulse to position the servo when triggered (a mono stable multivibrator). You need to switch separate resistor values on the monostable multivibrator for each servo position you desire.

Any sort of filtering would likely destroy the waveform needed to control a hobby servo, so that’s why it probably didn’t work.

All bets are off if your servos are not analog servos. There are digital servos as well.

We could design this for you, but might require $250 to$300 per hour for our work. If there is a pre-designed circuit available, it might take a lot of searching
There is a saying, picture = 1000 words

share a picture of the setup please
Also a picture of what your trying to do,
hand drawn is fine,

#### djsfantasi

Joined Apr 11, 2010
8,566
There is a saying, picture = 1000 words

share a picture of the setup please
Also a picture of what your trying to do,
hand drawn is fine,
Are you talking to me?

I’m not trying to do anything! The TS is… And I’ve posted a link to an article that describes the recommended circuit for the TS application. Look at it if you need a picture.

#### drjohsmith

Joined Dec 13, 2021
504
Are you talking to me?

I’m not trying to do anything! The TS is… And I’ve posted a link to an article that describes the recommended circuit for the TS application. Look at it if you need a picture.
It was aimed at the OP

#### djsfantasi

Joined Apr 11, 2010
8,566

#### drjohsmith

Joined Dec 13, 2021
504
Oh, you quoted my post so I was confused
My fault. I meant to put your name in.

#### JeremyOkulovich

Joined Jun 12, 2016
8
First, what do you want the servo to do when it detects the laser beam? What does it do when there is no light?

Detecting the laser beam depends on the ambient light. Personally, I’d start by measuring the photoresistor’s value with and without the laser beam.

What I’m suggesting is that you break your problem into two problems. Detecting the laser and controlling the servo.

With regard to the latter problem, do you understand how to control an RC Servo? Analog hobby servos are driven by varying the duty cycle of a 20 50Hz signal. It CAN be done with a single 555, but sometimes it depends on the particular servo you’re using. With a single 555, when you change the duty cycle to position the servo, you’re also changing the signal frequency. The off-spec frequency may not be recognized by the servo. This may be your issue.

IMHO, a dual 555 or 556 Servo Controller circuit (click on the link) is the best solution. The first 555 is used to generate the 20 50Hz signal with an astable multivibrator circuit. The second 555 is used to modify the duty cycle with a monostable multivibrator circuit. This circuit is what you would control with your photoresistor.

So you need to tell us about how you’re planning on using the laser. And do a little research on the circuit I provided.

UPDATE: Corrected mistake in signal frequency.
Hey finally had time to myself to mess with this circuit you send to me (the 555 servo controller circuit). It works but it only goes in one direction. So when I block the laser from hitting the photoresistor it moves in one direction, but when I unblock the laser so that the laser hits the photoresistor it does not go in the other direction. I would like it to move back to its original position when I let the laser hit the photoresistor. This is where Im having trouble in finding a solution.

Here is something that I did notice... When I turn the R7 potentiometer to completely to one side it will move the servo in one direction completely, but then when I turn the same potentiometer to 20% left of other side of the pot it will move the servo back to the original spot... how can I get this done automatically without having to turn the pot to one side or the other? This is what I am having trouble with.

#### djsfantasi

Joined Apr 11, 2010
8,566
I’m traveling, so can’t provide a detailed solution. But here’s the principle…

Measure the resistance of the potentiometer in both positions. Then, get two fixed resistors, equal to R7(min) and R7(max)-R7(min). Call them R7a and R7b.

Use the signal from the photoresistor to drive a transistor between the junction of R7a and R7b and ground.

Ta Da!

A picture is worth a 1,000 words - but I can’t make one now. But do you get the idea?

#### drjohsmith

Joined Dec 13, 2021
504
Hey finally had time to myself to mess with this circuit you send to me (the 555 servo controller circuit). It works but it only goes in one direction. So when I block the laser from hitting the photoresistor it moves in one direction, but when I unblock the laser so that the laser hits the photoresistor it does not go in the other direction. I would like it to move back to its original position when I let the laser hit the photoresistor. This is where Im having trouble in finding a solution.

Here is something that I did notice... When I turn the R7 potentiometer to completely to one side it will move the servo in one direction completely, but then when I turn the same potentiometer to 20% left of other side of the pot it will move the servo back to the original spot... how can I get this done automatically without having to turn the pot to one side or the other? This is what I am having trouble with.
It might be worth noting how the servo works
If I remember , and please it's a simple description. Some one else can full in I am certain.
It needs a pulse train , small pulses it goes one way , longer pulses it moves the other.
Pulse width modulation.
So if you start the servo in the middle you then make the pulses smaller to turn one way, longer to turn the other.
QED you always need pulses ,

#### djsfantasi

Joined Apr 11, 2010
8,566
It might be worth noting how the servo works
If I remember , and please it's a simple description. Some one else can full in I am certain.
It needs a pulse train , small pulses it goes one way , longer pulses it moves the other.
Pulse width modulation.
So if you start the servo in the middle you then make the pulses smaller to turn one way, longer to turn the other.
QED you always need pulses ,
drJohnSmith, I quoted an article earlier that described how RC servos operate. While in general you are correct, there is one feature you omitted. Varying the length of the pulses CANNOT change the frequency of the pulses.

It MAY work, but cannot be guaranteed for all servos.

Joined Jul 18, 2013
25,979
AGAIN the term Servo appears to be used indiscriminately, It means different things to different people.
I assume the OP is referring to a R/C Servo, it would help if this is stated at the outset!

#### djsfantasi

Joined Apr 11, 2010
8,566
AGAIN the term Servo appears to be used indiscriminately, It means different things to different people.
I assume the OP is referring to a R/C Servo, it would help if this is stated at the outset!
Max, I love ya’ man. And I know the membership is heavily oriented towards engineers. But nothing irks me more is the statement than the statement that “the term Servo appears to be used indiscriminately, It means different things to different people.”.

I dare say that most of the world are not engineers. And common sense makes it very clear that the term servo is being used for a very specific reason in most posts.

Or perhaps I am wrong in my assumption that engineers have common sense? If there is any ambiguity, common sense identifies that as an issue. No ambiguity dictates clearly what the TS is talking about by application of common sense.

The TS definitely is lacking in certain skill levels. The TS clearly is NOT talking about general servos. Membership apparent confusion as to what the TS is talking about, might be construed as trolling.

</endRant>

#### LowQCab

Joined Nov 6, 2012
2,506
People need to learn how to clearly communicate ideas to others,
and they certainly don't get taught how to do it in typical "Government-Schools".

If other people always try to "overlook" poor communication skills to make the other person "feel-better",
then the other person will never be presented with a challenge to improve their communication skills.

Continuously hammering someone to speak, and use language correctly,
is actually a gift to them, which will provide valuable dividends for the rest of their Life.
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.
.

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#### BobaMosfet

Joined Jul 1, 2009
2,028
Ive been trying so hard to find a schematic that does not have an arduino involved to control a servo with a laser. So far what I have is a laser pointing to a photoresistor that is the trigger for the 555 chip to create a pulse width. The pulse width was a bit too much for the servo so I tried to control it by putting in a low pass filter, but then the signal became too small (voltage wise) so I put the signal through an opamp and it is still not working right.
The breadboard and wires are a bit of a mess and would be hard to post a proper picture of it. If anyone knows where I can find a schematic of a basic idea that would be amazing. I'm trying hard to stay away from arduino because I would like to build everything from scratch so that I can try to really understand circuit design. Thank you for any help.
Title: Understanding Basic Electronics, 1st Ed.
Publisher: The American Radio Relay League
ISBN: 0-87259-398-3

Start with that. Use a transistor to control and LED into your photoresistor/photo-cell. Make sure you pull datasheets for the components, because you have to understand how to properly compensate for light/dark range with photocell depending on which you use, and what wave-length of light it is most likely to peak detect.

If you can do it with an LED, you can do it with a laser.