### Laser Galvo Control Experiments

I recently bought some laser galvos from Aliexpress. What I got was the basic guts of a laser light show system. It came with everything I needed except the lasers. My goal is to control a laser to make a large interactive artistic display or game controlled by a micro controller with a DAC. I have a PSoC 5 and an ESP32 that have DACs, so I may use those.

Galvos work like speakers, old school analog meters or hard drive arms. They move proportional to a current. They can move extremely fast and precisely with the ability to move to 30,000 positions per second. The galvos came with drivers. This simplifies the control to a simple differential voltage. A differential voltage is used to increase noise immunity. The ILDA spec says the differencial voltage is 0 to 10V. This means at one end of travel you supply -5V and 5V and at the other end of travel you swap the voltages supply +5V and -5V.

My DACs are 0V to 5V (0r 3.3V). I need to convert this range to a 10V differential voltage. The easiest way for me to do this is to use some op amps. With what I have on hand it is easiest to get the 10V differencial using 0 to 10V and -10V to 0. I tested on the galvos by outputting a -10V to 10V sin wave from a function generator. It worked great.

Assuming I use the 0-5V DAC, I need to map that to a -10V to 10V range. That is a 20V span and requires a 4x gain. I then need to offset that by -10V. The op amp circuit I show below is what I made.

I used an LM324N because I had some. That has (4) op amps in a single package. The rails of the device need to be hooked up to a + and – supply that is greater than the range I will be using. Fortunately the galvos came with a power supply that outputs +15V and -15V. I used some potentiometers in the circuit to allow me to adjust the circuit.

The lower op amp provides the gain. The gain on the positive side is 1 + 30k/10k which is 4. The negative side is used for the offset and its gain is 30k/10k which is 3. I need an offset of 10V, so 10V divided by the gain of 3 is 3.33V.  I need that value to come from the upper op amp. That op amp has a gain of 1 so I simply need to adjust the pot in front of that to output 3.33V.

The DAC is connected to the lower left voltage source in the schematic shown as 2.49V. The galvos would be connected to Gnd and the output of the lower op amp (green line).  The schematic is interactive. Click the link below and try adjusting the input voltage between 0V and 5V.

Click here to interact with the circuit.

I needed to create 1 circuit for each axis. Here is my breadboard.

Here are the signals on an oscilloscope. The yellow trace is the 0-5V input as a 5kHz sin wave. The blue trace is the output. Both traces use the center line as the 0V. The scale is 5V per division. The input has a low of 0V and a high of 5V. The output goes from -10v to 10v.

I am using the tiniest laser I could find so that I don’t have to worry about safety yet. I 3D printed a base to keep things in line.

Here is a short Instagram video of some testing.

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#### 6 Responses to “Laser Galvo Control Experiments”

1. very interesting project you have going on! I’ve wanted to do that for years!

I’m a bit confused about the ILDA spec though. first you say that it’s a 10V differential signal but later you say you need a 20V span.

According to the spec you link to the differential is 10V (page 11 for the X-axis) it says that:
‘When compared to the ground level of the
differential driver, the voltage level of the normal
signal line shall be +5V and the voltage level of the
inverted signal shall be -5V.’
To me this implies that you need a +/-5V range and not a +/-10V as you wrote.

As long as you provide both positive and negative voltages for the opamps you wouldn’t neet to go above +/-5Vin the output or am I mistaken?

2. Yes, you are correct.

I was skipping the control card on my initial tests and going straight to the galvo driver. That has a larger voltage range. The circuit can be adjusted for other ranges.

https://www.aliexpress.com/item/30K-laser-Galvo-Galvanometer-Based-Optical-Scanner-including-Show-Card/32828525350.html

3. FreddYokel

Any chance of a copy of your 3d printed mount?

4. Bruce

Hi, I am a little confused about your schematic. The +/- 15V supplied to the opamp is where I am fuzzy. My 15V supply has a wire for + and a wire for – and a wire for ground. Are you wiring both the positive and negative wires to the op amp input? And is your output from the circuit get wired to both the + and – of the driver?

5. @Bruce

Not shown on the schematic are Pin 4 Vcc+ and Pin 11 Vcc-. That is where you attach the + and – from the power supply.

If you look carefully at the breadboard image, you can see the wires going to the middle pins on each side of the I.C.

6. Flex Bex

Hey I just got the same package from Aliexpress. Have you considered to simply replace the controller chip. I saw the AD7528 on mine. So when I take out the large STC controller chip I should be able to somehow access the DAC from the now open pins where the controller chip was before. Which would be good as my Launchpad/RPI don’t come with a DAC.