Well, I honestly thought i'd cracked it!
The two 741s work ok, but I need devices that can get much closer to ground in order to ensure my optocoupler is turned off. So, move over to the LM358 - gets very close to ground, and so cheap theyre practically giving them away from China! 100 devices ordered, 50/50 DIL and SOIC, for £2.47. And since I found £2 abandoned in the gym lockers, net cost to me 47p!
Being dual rather than single devices, I thought i'd also got it down to an incredibly tiny little circuit...
...only thats not the case. My circuit relies on the output on the 1st op-amp controlling the ground pin of the 2nd. Only trouble is - a dual device only has one set of supply pins!
So at the moment im still stuck using TWO physical devices, leaving me with two unused op-amps on the board. The only other thing that is preventing me getting really small is the preset resistor. I reverted back to this with the idea of selling a few of these devices to make back the development cost, and thought that the end user would probably prefer to have the threshold voltage adjustable. And at least in the PRC344, space is an absolute premium, and the board has to be physically as small as possible! There are of course SMT preset resistors, but ive yet to find them in the EDA library, so at present cant design a board to use them! That would be ideal and i'll ask on the forums for a package file. The other alternative is to go back to fixed resistors and let the user decide their own values.
At present with the standard PCB preset and two SOIC LM358s, ive got the PCB down to 27mm x 17mm. Thats not bad, but is still a tight fit in the only 'safe' places in the radio.
So, making the advance decision to go back to fixed resistors, my limiting factor is the two ICs. How do I get around this and down to a single IC?
Ive done some playing, and theres no easy way to simply use the output of one op-amp to control the other via its inputs. So perhaps thats not the way to go? Rather than trying to control the op-amp pulse generator itself, maybe just control its output getting to the optocoupler? The detector op-amp could switch a transistor in the cathode of the opto. This would mean that the pulse generator was running continuously, but if theres no load on the output, is that a problem? How much current does it draw with no output load?
Time for some measurements and more testing on the breadboards!
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