Rich Grise wrote in news: snipped-for-privacy@example.net:
Wrong...wrong...wrong! Google "PicAxe". Free PBasic programming software, program loads from a serial port directly (through two resistors) to the chip, and 8 pin chips are cheap (even with shipping from GB). They were intended for teaching uP's in elementary and high schools, but are great for real world projects. I have several controlling science displays. These are "PIC's" with an onboard PBasic interpreters. The 8-pin, 4MHz PicAxe08/08M will easily do what you described and more.
Right- the 555 pin 5 ,CONT, is the first node of an internal R-R-R voltage divider to GND which is the threshold for the THRESH switch point, and 1/2 the CONT voltage is the threshold for the TRIG switch point. By connecting a small diode from CONT to OUT with anode at CONT, when OUT is high, the diode is back biased and does not affect the trip point for THRESH, but when OUT is low, which means the timing capacitor is being discharged downwards towards the TRIG threshold, the diode pulls the internal CONT node down to 0.7V making the new TRIG threshold ~0.35V. In your application, when you remove RST, the first beep lasts for the time it takes the timing capacitor to charge from 0 to THRESH threshold of 2 x Vcc/3, then goes off for the time it takes to discharge to 0.35V, then back on for the time it takes to re-charge from 0.35V to THRESH threshold, then off for time to discharge back 0.35V, etc...All the ON times are about the same for practical purposes this way. Without the diode the second and third on times would be of duration the time it takes to charge from 1/3 Vcc to 2/3 Vcc- and they sound noticeably shorter than the first beep.
When a last input is removed, the input NAND gate RSTs the 4017 and drives the base of the 2N3904 all at the same time. The RST of the 4017 removes its base drive from the transistor tending to turn it off while the NAND is applying its drive turning it ON. The NAND base drive part should be solidly on several hundred nanoseconds before the 4017 outputs begin to remove their base drive so that I am at a loss to explain why you are hearing a blip. If you are manually removing a clip lead or using a switch, which has bounce at turn-off, then an intermittent disconnect-connect-disconnect etc jittering of the input contact, however brief, may be causing the blip you are hearing. You can eliminate the effect, for reasonable jitter of input connection times, by adding a filter capacitor from collector to base of the 2N3904. This will make the transistor have a fast on- slow-off characteristic and help prevent the blip. A schematic with all the changes:
Ok, yeah I thought the case. I'll ammend my printed copy. The inputs may or may not be coming from other logic gates, so it's best I connect the 1M drop-downs I think.
Your modification below actually worked a lot better than the original (I decided to go with the modified schematic because I didn't have a CMOS 555 handy). It's working more normally now =) - I am wrapped! But there's just some very minor issues now that I hope you could sort out for me...
The first beep appears to be longer than the second and third (I know you mentioned something earlier about a diode, but I didn't quite understand). I take it that's because the 555 has just come out of a reset? So as a result, if I connect, say, the 3rd INPUT for three beeps, I hear: beeeep! beep! beep!
With the 10K resistors connected in series and also the 1M drop-downs for each input like you said, once I disconnect an input (after beeps have finished), I would hear a very brief 'Blip!' from the buzzer. I actually thought maybe the 1uF cap had something to do with it, but still does it even if I remove it.
Oh, at power on with an input already connected, it is still beeping 6-9 times. > Not a big issue however. Mainly concerned with the above two.
It works! Hooray! Thank you so so much. You have been great.
The diode worked very well to fix the longer beep problem -thank you-, but as with the cap to fix the 'blips' - it failed miserably... actually made it worse. So I left it as it was, but connected a transistor to each input and works a lot better now.
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