Project: Discontinuous Interference Analyzer System Test Circuit

Thread Starter

Bill B

Joined Nov 29, 2009

After a lot of on again off again work on this circuit I finally have it working. A while back, at work, I discovered the discontinuous interference (click) analyzer hadn't registered anything for several consecutive tests. I used a signal generator and 555 timer circuit to troubleshoot the system and discovered we had a broken input cable inside the analyzer. A quick solder job and it was back up and running. The boss decided that we needed a permanent system to verify our setup before testing.

The click test measures short noise events on the power mains that are generated by a tool or appliance connected to the circuit. These noise events are characterized as short clicks (<10ms) or long clicks (>10ms). There are four channels monitored by the system, 150kHz, 500kHz, 1.4MHz, and 30MHz. I needed to generate these 4 frequencies and switch them on and off sequentially for a specified duration with a consistent period of time between switching events.

For the 3 lower frequencies I used 74HC14 Schmitt trigger inverters with an RC feedback network to set the frequency. For the 30MHz signal I used a canned crystal oscillator. Each oscillator output was connected to an input of a 74HC00 NAND gate. I used a PIC12F629 micro to enable the NAND gates for 8ms, 17ms, and 25ms (click durations) with a 10 second interval in between each click duration. I used a series base resistor and potentiometer on each output to control the amplitude and series caps on each output for decoupling. The outputs are all connected to a 2n4001 transistor configured as an emitter follower buffer and then fed to a pair of X-caps that couple the noise onto L1 and neutral of the power mains. I generated the micro programming code using Flowcode. The best part of this project (especially where management is concerned) is that I was able to use parts that were laying around the lab to build it with the exception of the PIC micro, which is cheap anyway.

Attached is a picture of the board, final project, source code, and schematic. Thanks to all who provided advice and support on this project.


Last edited by a moderator:


Joined Dec 17, 2014
These are amateurish questions for you, I'm sure, but I hope you don't mind.

Are the X-caps (CY1 and CY2 ?) a special kind of capacitor, or would any 10nF cap with a high enough voltage rating do the job? If you send the high frequency bursts onto both the hot and neutral wires wouldn't they cancel?

For the 50 Hz mains voltage, CY1 and CY2 are a high-pass filter preventing the mains voltage from destroying your circuit, is that correct?