Analog & digital modulation for laser pointers

Fig.1: the analog modulator uses IC1 & Q1 as a unity-gain amplifier, with VR1 setting the DC output level.

There are a number of ready-made 1mW red (630-650nm) laser pointer modules available on the market (DSE, Jaycar, Oatley Electronics, etc) but the documentation is sparse and provides no stated way to modulate the light output. However, it is possible to get usefully modulated laser light beyond 80kHz using these modules and a handful of external components.

A typical laser module has a minimum supply voltage (1.9V in the units tested) at which light begins to be emitted. The maximum supply voltage and light output occurs at 4.7V. Supplying the module with a varying DC voltage between these values produces a varying light output.

On the reception side, detectors for laser light fall into two groups. Phototransistors have a rise/fall time of around 3μs to 10μs. Photodiodes have a higher ft but in general are designed for infrared wavelengths and so have a poor response at 650nm.

Fig.2: the digital modulator. Q1 provides a constant voltage to the laser module, while Q2 & diodes D1-D3 allow TTL or CMOS drive signals.

Fig.1 shows the analog modulator. IC1 and Q1 act as unity-gain inverting amplifier (ie, a buffer), with trimpot VR1 acting as a level shifter to provide a permanent DC component. VR1 is set to provide 4.0V quiescent at the laser module’s positive terminal. A 1V peak-peak sinewave of varying frequency was applied to the input capacitor and the light output measured using a BP103 phototransistor (Fig.3).

Light falling on the phototransistor acts as base current, turning the transistor on. The value of the series collector resistor Rs changes the sensitivity of the device. Increasing Rs makes the unit more sensitive to low light conditions, decreases the speed of operation, and increases the possibility of the transistor saturating (which further decreases the speed of operation).

The power rating of the BP103 is stated at 150mW, which limits the value of Rs at a supply voltage of 5V to a minimum of 42Ω. The overall frequency response of the combined modulator/detector assembly is shown in the graph (Fig.4).

Fig.3: the photodetector circuit.

The light output was sufficient to saturate the phototransistor despite the low value of series resistance, so an optical attenuator (consisting of several layers of translucent plastic) was placed in the light path to provide a quiescent output from the photo-transistor’s collector +2.5V.

Note that the response time of the detector may be the limiting factor in the assembly’s response. Lacking a fast phototransistor, I suspect that the module does top out at around 100kHz. when a 3:1 mark:space ratio pulse train is applied to the module and the frequency increased, the beam becomes appreciably brighter after 150kHz, suggesting that the module is not turning off to the same extent as at lower frequencies.

Digital circuits do not require a linear response, just an obvious difference between on and off (the more obvious, the better). The digital modulator (Fig.2) looks a bit like a 2-transistor differential amplifier but it is not. The 500Ω trimpot VR1 and Q1, a BC547 NPN transistor, provide a constant voltage to the laser module which sets the minimum brightness.

Fig.4: this graph shows the response of the analog modulator/detector assembly.

Q2 and the associated diodes allow TTL or CMOS circuitry to drive the laser modules. A PNP transistor is used here because TTL can sink quite a bit of current but cannot source nearly as much.

Adjust trimpot VR1 with the TTL input disconnected (or high) to give about 2.4V on the module. The exact value depends on the module being used. Diodes D2 & D3 can be omitted when using CMOS or microcontrollers which switch to the supply rails.

The laser module came from Jaycar (ST-3115, $13.95), and the BP103 came from Oatley Electronics (part of the $10 Opto-Pack). Jaycar has a similar device to the BP103 (ZD-1950, $1.00).

The author would like to thank Doug Baly of Ultimo TAFE for his assistance with this project.

Measuring the current drain of devices with DC-DC converters It is often not possible to measure the current drain of modern consumer portable electronic devices by simply connecting an ammeter in series with the battery. Such devices, including digital cameras, digital notetakers, portable MP3, minidisc and CD players, often use only one or two AA or AAA cells and employ a DC-DC converter to step up the available voltage. The waveform of the current they draw from the battery is a high frequency square wave with the peak current that’s several times the average current. With such a high peak current demand, connecting even a relatively low resistance ammeter in series with the battery will often reduce the peak available current by so much that the device will refuse to operate. The situation is complicated by the presence of sophisticated battery condition monitoring circuitry in many of these devices. This circuit avoids these problems. A dummy battery replaces all of the cells in the device under test and a variable voltage power supply provides the power for the device. A 6800μF and a 0.1μF capacitor are wired directly across the terminals of the dummy battery to ensure a low supply impedance at both high and low frequencies. The ammeter and voltmeter (which can be multimeters set to the appropriate ranges) monitor the current drain and actual supply voltage under load. By slowly reducing the voltage from the power supply, the device’s behaviour can be observed as the "battery" runs down. Some devices will shut off when the supply voltage is low (but not zero) and not respond even if the supply voltage is increased to normal – after all, batteries do not usually spontaneously recover their charge! This is why the switch is included in the circuit. Setting the switch to the ‘off’ position simulates the removal of flat batteries. Then the power supply voltage can be set to that expected from a good battery and the switch set to ‘on’ to simulate the insertion of fresh batteries. Andrew Partridge,Kuranda, Qld. ($30)