Servo - October 2011

FIGURE 4. I used this custom development platform for the majority of the project.

Laser FAQ ( www.repairfaq.org /sam/ laserdps.htm#dpstoc) has a bunch of schematics of varying complexity. The advantage to this solution is price — you can get by with a handful of parts for or under

$1 in quantity. However, creating a laser driver with a discrete APC solution that actually works reliably in production capacity can be a challenge.

2). Integrated circuit (IC) driver. An IC driver contains the necessary drive circuitry, APC, and laser diode protection mechanisms in a single package, though an external laser diode and collimating lens are still required. A very attractive part is the iC-WK universal laser driver from iC-Haus ( www.ichaus.de/product/iC-WK%20iC-WKL). The iC-WK comes in an eight-pin SOIC or MSOP package, provides support for all three common laser diode configurations (N-type, P-Type, and M-type), and provides CW drive current up to 90 mA. Ballpark pricing for the iC-WK is ~$2.70 for 1K pieces. An evaluation board was readily available which allowed me to get up and running in minutes (all I needed to do was adjust the maximum allowable output power using a single resistor). A disadvantage of this solution is that I would still need to purchase a discrete laser diode module with integrated collimating lens.

3). Laser diode with integrated collimating lens and onboard APC. Known as an "APC laser diode" and developed by Arima Lasers Corporation in Taiwan ( www.arimalasers.com/en/products02.aspx), these units contain a laser diode, plastic or glass collimating lens, driver, and APC circuit on an ASIC — all built into a single three-lead package (with only two of the leads actually used). Lots of different sizes, laser diode wavelengths, output powers, and collimating lens options are available, and pricing ranges from around $2 to $8 in 1K quantities depending on those options. The laser diode is simply additional circuitry is required. The advantage here is price and integration, since the entire unit is in one package.

During my evaluation, I kept my eye on three important criteria: • Ease of use.

I ultimately selected Arima’s APCD-635-02-C3-A laser diode — a Class IIIa device with a maximum power output of <= 3 m W at 635 nm. It has a metal housing with an integrated glass collimating lens. The cost of the laser diode ($7.55/1K) is essentially double the cost of the 650 nm wavelength version ($3.65/1K) that is more commonly used in low cost laser pointing devices. However, the OVM7690 — like most camera modules — contains an IR elimination/cutoff filter (more recognizable to engineers as a low pass filter) that blocks infrared and passes visible light.