01-02-2016, 11:25 AM
Thanks Ref, hadn't thought of using a wallwart. That's a nice KISS solution for what will be a "one off". It also means I don't have to make sure the mains side wiring is safe. I'm not worried about response time in either direction so the decay time of the wallwart output wouldn't matter. I'd use a series resistor to limit the current in the CMOS catch diodes. possibly have my own catch diodes too.
Mark, your thought of a 10K series resistor before the CMOS is a good one. The diodes could easily end up as 1N4148 anyway. I really should have a reverse polarity diode from the 22M resistor to 0V. This means that on -ve half cycles the input to the rectifier is defined, rather than possibly up to full mains. In practice it won't get anywhere near full mains because of stray C but better to be certain.
In case you're wondering what all this is about, it's for the AV router at the Museum. When the head end system is switched to standby the outputs need to go to defaults. The standby switch removes mains from the COW, DVD player and monitoring, leaving the freeview boxes, RF distribution and Aurorae active. The defaults are currently off-air on all 3 channels. The new system will be switchable from off-air to testcards by an extra switch on the control panel.
Assuming I use the Invotron routers I've devised a cunning circuit using a couple of 4051 analogue 8:1 muxes to do the defaults. Soon I'll draft the schematic properly in Orcad. Makes it easier to build when you've got a properly annotated schematic in front of you and also makes sure it's documented.
Mark, your thought of a 10K series resistor before the CMOS is a good one. The diodes could easily end up as 1N4148 anyway. I really should have a reverse polarity diode from the 22M resistor to 0V. This means that on -ve half cycles the input to the rectifier is defined, rather than possibly up to full mains. In practice it won't get anywhere near full mains because of stray C but better to be certain.
In case you're wondering what all this is about, it's for the AV router at the Museum. When the head end system is switched to standby the outputs need to go to defaults. The standby switch removes mains from the COW, DVD player and monitoring, leaving the freeview boxes, RF distribution and Aurorae active. The defaults are currently off-air on all 3 channels. The new system will be switchable from off-air to testcards by an extra switch on the control panel.
Assuming I use the Invotron routers I've devised a cunning circuit using a couple of 4051 analogue 8:1 muxes to do the defaults. Soon I'll draft the schematic properly in Orcad. Makes it easier to build when you've got a properly annotated schematic in front of you and also makes sure it's documented.
www.borinsky.co.uk Jeffrey Borinsky www.becg.tv
