| -100% Peak Modulation Detector
Circuit Description:
An NE555 timer is used to convert a very brief trigger to a more visible 0.5 second indication. Power for the circuit can come from any convenient supply but should be limited to 12 volts at the IC for proper peak detection. I use this in a Viking Ranger, so the 6 volt A.C. heater supply and a #51 bulb were chosen. You can also substitute an LED with the appropriate dropping resistor, such as a 2.2k, which works well when powered by a 6 volt heater circuit. The 470k resistor and 1 mFd capacitor set the "on" time for the indicator. I found 1/2 second to be good for catching the eye with an incadescent indicator. Lower either value for a shorter time constant.
On the input trigger side, the 5 volt zener diode, D2 (1N914), and the 1M resistor ensure that the trigger voltage at the input of the IC remains between 0 and +5 volts. Either point A or B would be connected to your modulated B+. Both work well, so it's up to you.
At A, choose the resistance and wattage of R1 to drop the modulated B+ sufficiently so that the maximum current rating of the protective zener diode is not exceeded on positive modulation peaks. The indicator is triggered when B+ drops below 5 volts, which is -99% for a 500 volt supply.
If B is used, no modulated voltage is passed by D1 until it drops below 5 volts. R2 is used to protect D2 during excessive negative voltage peaks. The value is non-critical and can be between 1 and 10k depending on your supply voltage.
|
There's a thin line between being loud and splattering (only a slight percentage of modulation) and this simple circuit will help identify when that line is being crossed.