School of Physics - Optics Group

Laser shutter using a hard disk drive voice-coil actuator

We have developed a high-speed laser shutter based on voice-coil actuators found in generic hard disk drives. The shutter is described on our shutter web page. Here we describe the circuit contained in our paper, Maguire LP, Szilagyi S and Scholten RE, High performance laser shutter using a hard disk drive voice-coil actuator, Rev. Sci. Instrum. 75 (9) 30773079 (2004).
The circuit in our paper, designed by Sandor Szilagyi, provides very fast switching, a small holding current, and bidirectional drive from a unipolar supply.

Unfortunately, an error was made in transcribing the circuit into journal format, such that in the paper, Q2 and Q4 are shown inverted, i.e. with drain and source reversed. The correct circuit, and PCB layouts, are included below.

Please see our new circuit for a much simpler circuit which works well.

If you would like to use the old circuit (which has better control of fast pulse duration), we recommend replacing the old (discontinued) FETs with MTP2955 for Q2, Q4 and MTP3055 for Q3, Q5 in Fig. 2 of our paper. These FETs are only about 40 cents each. Note that the reed relay can be replaced with a standard relay, as shown in the photo below, although reed relays are generally faster and not expensive.

Schematics and PCB layouts

Note that you really only need a single-layer PCB: just etch the bottom layer, and drill, and add four patch-wires to take care of the top layer.

The board size is 3.00" x 2.40" (76mm x 61mm).

If you would like to purchase a milled PCB as in the photo, without parts, our workshop will be happy to help out; estimated cost will be about A$60 to $70 each. They can also assemble and test one for you, but the cost is $90 per hour.

How does it work?

The core is the MOSFET H-bridge (Q2 - Q5), which drives the unipolar supply through the coil, one way when Q2 and Q5 are switched on, and in reverse when Q3 and Q4 are switched on.

Assume the input TTL signal is low, holding the Q1 gate voltage low, switching Q1 off. The potential on the drain of Q1 rises close to +30V. Current flows through D3 when the voltage across D3 is above 18V, raising the gate-source voltage on Q3, switching it on. On Q2, the gate will be close to +30V, with D2 not conducting. Thus the left-side of the H-bridge will be shunting current to ground.

Q6 inverts the input TTL signal, so that on the right-hand side of the H-bridge, the junction of D4 and D5 will be at low voltage, thus there is more than 18V across D4 and it conducts, raising the gate-source voltage on Q4 such that Q4 switches on, conducting current through the coil (and Q5 is off).

When the TTL input switches high, Q1 turns on, reducing the voltage on the drain. The high-to-low transition propagates through the first 22nF capacitor, producing a short positive spike on D8 which briefly turns Q7 on, driving the relay which shorts the 220 ohm resistor, thus giving a large current spike to the coil. The reverse happens on the second 22nF cap because of inverting Q6, but D7 isolates the negative spike from the gate of Q7.

When the TTL input switches low, the same thing happens via the second 22nF cap and D7, again allowing a large current pulse through the coil as the H-bridge reverses.

You may need to change the two 22nF capacitors to suit your relay and voice coil. Ours are currently 47nF.


Created: 10 March, 1999
Last modified: 25 August 2006
Authorised and maintained by: A/Prof. R. Scholten, School of Physics