Make Your Own Solder-In Passive Oscilloscope Probes
This DIY solution makes it easy to probe multiple points in a circuit with a scope.
Oscilloscopes can show so much of a circuit's behavior that they are an engineer's favorite tool. But, before they can provide insight, signals in the circuit need to be probed. Shabaz on the element14 Community shows how to build solder-in passive probes that are inexpensive and highly functional.
"This blog post may be useful for those who need low-cost ‘scope probes, or for those who want solderable probes for attaching to a circuit board, temporarily or even semi-permanently." -shabaz
Most oscilloscopes come with passive probes that attenuate a signal by a factor of ten. We call them "passive" because resistive, capacitive, and inductive elements make up their construction. Active probes, on the other hand, have a field-effect transistor that buffers the signal.
A vital benefit of a 10:1 probe is that it is very high impedance. However, they can be challenging to attach to a circuit permanently. Most of these probes do not ship with a "solder-in" accessory.
DIY solder-in passive probes make it possible to attach multiple probe points to a circuit easily. Unlike the 10:1 probe, these have an input impedance of only 1 kilohm. While low compared to a traditional passive, it is suitable for probing digital signals.
Constructing the DIY oscilloscope probe is straightforward with shabaz's instructions. The supplies needed are a piece of RG-178 coax, a BNC connector, a 950 ohm resistor, some wire, and hot glue. An optional but helpful material is polydoh.
In short, you peel back the coax's outer braid, crimp it, and solder a surface mount resistor to the center conductor. From there, a small piece of wire, ideally color-coded, and some hot glue hold the soldered structures together.
For additional mechanical strength, shabaz used polydoh. This moldable plastic comes in granule form and can be re-shaped once heated. After cooling, they are stiffer and less sticky than hot glue. Also, the granules can accept pigment to color-code the wires even more.
Performance of the DIY solder-in passive probe is on part with an active FET probe. At least, if you take into account the difference in loading. FET probes tend to be in the hundreds of kiloohms, and this solder-in passive is only about 1 kilohm.
One requirement to note is that these probes require on oscilloscopes with a 50-ohm input impedance. The 950 ohm tip resistor will not work well with a 1 megohm-only oscilloscope front-end.
For detailed build instructions, waveform comparisons, and plenty of pictures, check out shabaz's DIY solder-in passive probe on the element14 Community.