Nuts & Volts - February 2006

■ FIGURE 4a. Project’s top view.

shadowing that of a nine-volt battery — this project uses both leaded and Surface Mount Technology (SMT) devices, which are evident in the photos of Figures 4a and 4b, respectively. I chose moderately-sized SMT devices, so as not to tax your soldering skills too much. Please note that for ease of assembly, the SMT devices must be soldered first.

A note on capacitors C1, C5, and C6: These are 1 microfarad devices, and they are available as either a ceramic or electrolytic type. You can use either one but the ceramics are the preferred devices. However, if you choose an electrolytic, pay attention to the polarity signs. C3 and C7 are (because of their value) found almost exclusively as electrolytics, so do pay attention to polarity.

Since the circuit has so much gain, it is important to isolate the microphone from the circuit board. Do not install it flush against it. Rather leave a 0.1” to 0.2” gap, and then fill that gap with hot melt glue for mechanical stability. Speaking on the subject of microphones, you may wish to mount it remotely. If you do so, please remember that electret microphones are polarized. The square pad on the board is positive. Use shielded cable for the hookup. Please note: This project is not designed to work with dynamic type microphones.

The board connects to the external flash with hookup wire to the flashes’ hot-shoe via an inexpensive adaptor (see Figure 5) or, in some instances, the flash may have a phono plug for external triggering (see Figure 6). You may find out that many times the flash units or hot shoe adaptors

■ FIGURE 4b. Project’s bottom view. PCB is not much larger than a nine-volt battery

have a coaxial type connector called a PC connector, somewhat unique but very common to the photo industry.

This connector is clearly visible in the hot shoe adaptor photo. Prior to attempting to trigger the flash with this circuit, you must determine the polarity. Do this by powering-on the flash unit by itself, then connect the hot shoe adaptor and identify the (+) and (-) leads with a multimeter in the volts range. Then connect them to the proper PCB pads, W3 and W4, respectively. Likewise, the battery’s (+) and (-) leads connect to W1 and W2.

Testing the Circuit

■ FIGURE 5. Flash hot-shoe adaptor.

Photo courtesy of B&H Photo and Video.

To initially ensure that the circuit is working properly, you do not need a flash unit or a camera. After you have double-checked parts placement and the soldering (especially important for the SMT devices), connect the nine-volt battery, set the potentiometer at maximum gain, and clap your hands in front of the project. You’ll see the LED flash.