Clear all

Forum 141

[Sticky] Simple LOPT ringer

1 Posts
1 Users
1 Reactions
Posts: 136
Estimable Member Registered
Topic starter

Hello all,
Whilst in the process of repairing my little GEC TV, it was suggested that I check the LOPT to make sure it was OK. As usual, I prefer to build these things, than buy them, so I set about designing a LOPT ringer.
Seeing as there were a few 555 timer ICs in the parts box, I chose to utilise one of these.
My design has a diode protected (and limited) output. After some experimentation, I settled on 100us pulses spaced at 50ms intervals. Anything more than a pulse of about 150us and the LOPT would react to the initial rise of the pulse and create a fair old bit of hash on the waveforms.
The old Veroboard came in handy again, so I won't bore you with the details of that!
So, below is the circuit diagram for the LOPT ringer of Boing!

Schematic LOPT ringer of Boing Sheet 1 20180903224329

Here's the waveform of the initiator pulse, 50us and 200mv per division.

Ringer 100us pulse

Here's the pulse train, 10ms and 200mv per division.

Ringer 50ms pulse

Lastly, here's a waveform of a known good LOPT (it's the one on my little T-9 portable). [Edit: 50us and 200mv per division]


If anyone finds this useful, then all the better πŸ™‚
As usual, if you have any comments or questions, please don't hesitate.

All the best,


Standards are like toothbrushes. Everybody needs them, but nobody want to use yours!

Posted : 03/09/2018 10:38 pm
6.3volts reacted