Scope Probe

Posted on Aug 11, 2012

Working on live telephone circuits can be a problem. You just can't connect a normal scope probe to the circuit while it is tied to the phone line. Neather side of a phone line is ground. When a phone or other device is 'offhook', one side floats at about -22 volts and the other side about -30 volts. This probe allows connecting to such 'floating' circuits to a normal scope input. As seen in the circuit, it is based on using an optocoupler. The one shown in the diagram is a dual unit, but I had a pile of them here, so thats what I used. Because this device responds to both polarities, an external diode was added to prevent negative voltages from producting an output.

Scope Probe
Click here to download the full size of the above Circuit.

The input has 2 ranges: 1 to 1, and 10 to 1. There is no need for a power switch because with no input, virtually no power is drawn from the 12 volt (keyfob) battery. Also, no power is drawn from the 1.5 volt (watch) battery. Because of variations in individual optocouplers, you should select a value for R3 to get the best results. Now for the bad news..... This device is enherently non-linear. The small battery on the input side helps compensate by giving an initial bias to the optocoupler's input LED. Also, presenting a 10k load to some circuits will cause addional error in the output. The non-linearity is not too bad from 0 to 5 volts, above that, it is not very usable. Nonetheless, this little gadget has already 'paid for itself' in making some timing and voltage measurments of how a circuit responds to a telephone ring signal.

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