Nuts and Volts - November 2013

November 2013 21

QUESTIONS and ANSWERS

Video Monitor Alarm

QI have been reading Nuts & Volts for over 13 years now and have greatly enjoyed the Q&A section. I now must pose a question that I’m hoping you can answer.

I’m using a 1.2 GHz 30 IR LED wireless camera and receiver to monitor my backyard both day and night. The receiver has two RCA

output jacks: audio and video. I am looking for a circuit that can generate a trigger from the video output when something new comes into the field of view. The trigger would either be used to activate a recording device or visual signal such as an LED. I have an MC9S12 Dragon12 development board and have been trying to use the analog-to-digital converter input to monitor the analog video output from the receiver. — John Foster

AI am sure that it is possible to do what you are attempting using digital signal processing, but I’m an analog engineer and have no idea how to do it. What I would do is rectify and filter the analog video signal and use a window comparator to set the alarm if the level went higher or lower. The time constant of the filter would need to be short enough that the change from daylight to dark would not cause an alarm, and some way to deal with passing clouds would be needed. The DSP (digital signal processor) would need to deal with those issues also. A better solution in my mind is to use a motion detector — preferably one based on heat so that moving branches due to wind will not be detected. AI have reproduced your circuit here (which was in the September issue) for clarity in Figure 1.

The smaller coil apparently stores less energy but the field collapses faster, so it produces a higher voltage but less current.

If you use a 555 one shot for your switch circuit (you will find the circuit and instructions on its datasheet) and power the U1 circuit from the 555 output, the power increase will be minimal.

I doubt that your circuit is stressing the 12V 5A supply, but more voltage input will give more voltage output. However, you run the risk of internal breakdown of the insulation of the coil if you exceed 14 volts at the coil. I assume that you are using an oscilloscope to measure the voltage loss at the coil because an analog meter will only measure the average, which depends on the duty cycle; a digital meter won’t give any useful reading.

The waveform at the drain of Q1 should be a ramp as the current in the coil increases with a sudden rise to two volts as the coil saturates. You don’t want the coil to remain in saturation because it just wastes power and heats Q1. Reduce the pulse width to minimize the time in saturation. You can measure the duty cycle and the average current from the source and calculate the peak current; see Figure 2. For excellent reliability, the voltage rating of electrolytic capacitors should be double the applied voltage, but for hobby use you can relax that because the caps normally exceed their rating and a failure is merely a nuisance. ■ FIGURE 2.