Signal Injector. Highlight Pen.
 

Had a little time to waste the other day and spied a High Light Pen in the bin.
Using a couple of BC548 transistors and a few other components on a matrix board driven by a CR2032, works a treat.
John

Re: Signal Injector. Highlight Pen.
 

Very neat job.

I built one in an old remote control.

Re: Signal Injector. Highlight Pen.
 

That looks a very useful item and a good idea recycling the pen case.
The rectangular section of the highlighter pen lends itself for various probe assemblies and could also be useful for a signal tracer, RF probe, logic probe, logic pulser, in circuit transistor tester etc.

Regards,
Symon

Re: Signal Injector. Highlight Pen.
 

I would have gone one stage further and put a tactile switch behind the probe tip so that when you press it on a test point the switch powers it up.

Re: Signal Injector. Highlight Pen.
 

I've a selection of saved highlighters, possible projects in addition to injector:
KV probe for 10M DMM using 33M HV resistors, a longer highlighter.
Peak detector 1nF series, 2 x 1N60, 100K load and 1nF. Feed wobulator in, scope or DVM.
L.O. or IF sniffer, basically very low capacitance FET buffer so as to not pull off frequency.
Voltage probe / battery tester with bargraph IC, high impedance or 470 Ohms.
Electrometer probe, either Rod pentode (robust) or FET to drive DMM, more than 100M input impedance.
RF probe, logic probe, logic pulser already mentioned.

Also useful and saved are the metal tip flat highlighter style white correction pens. I suspect the dremel cutting wheel will cut of the ballpoint tip if it's a problem.