SHOWALL FUN PRANKS HIGH VOLTAGE LAZARUS-64 PHOTOGRAPHY SPY GADGETS VIDEO GAME
Figure 2 - This is the schematic for the sound activated camera trigger
Figure 2 - This is the schematic for the sound activated camera trigger

Looking at the schematic shown in Figure 2, you can see that the output from a small microphone is sent into the LM358 op-amp, which acts like a comparator, responding to changes in voltage from the microphone. Because the microphone is of the electret type, it includes its own high gain amplifier right inside the tiny metal can, further increasing the sensitivity of the comparator circuit. The variable resistor VR1 controls the sensitivity of the comparator so sounds as faint as a whisper or as loud as a clap can be used to activate the camera switch relays.

The output from the op-amp comparator is sent into a signal conditioning IC called a one-shot. The 74121 one-shot will take any input voltage, no matter how short of a duration and send an output signal of some calculated time based on the values of resistor R3 and capacitor C1. For the camera trigger, a time of about 1 second is fine, so the values of 100uF and 10K set this time. The reason it is necessary to have the one-shot in the circuit is because the signals from the comparator will be modulated by the sound, and will likely be too fast to allow the relay to close long enough to trigger the camera shutter release. The one shot simply lengthens the pulse, which also gets rid of multiple pulses from the same quick sound. Since the camera needs to respond quickly in this application, both the focus and shoot relays are tied together, allowing the image to be taken as fast as the camera can respond to the sound. A very fast camera would be able to perform stop motion photography, like the capture of a balloon popping or water drop.



Figure 3 - Part of the datsheet for the 74121 one-shot IC
Figure 3 - Part of the datsheet for the 74121 one-shot IC

The 74121 is an interesting and very useful IC, as it can create a pulse varying in length from nanoseconds to 30 seconds by varying the values of the capacitor and resistor used for timing. By having such precise control over pulse widths, very accurate timing systems can be made, although in this application, the 74121 basically acts like a switch debouncer to simulate the operation of a human button press. The datasheet (Figure 3) shows that the switch is controlled by a pair of timing pins and has multiple inputs to allow different pulse edges to create the output. In this circuit, the one-shot is set up to send out a 1 second long digital pulse every time the input goes low. The input is the AC voltage signal from the op-amp comparator that is responding to changes in sound picked up by the microphone.

Back Home Last Next
You are Viewing... Page 2 of 6
Lucid Science Electronics from the Fringe AtomicZombie Hack-a-day SparkFun