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jayxflash
Joined: 28 Nov 2003
Posts: 8
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 08 Oct 2004 16:24 problem with 7474 bistable |
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| I have a 7474 configured as bistable, and I don;t know why the circuit is not working. the scheme is presented here: http://www.zen22142.zen.co.uk/Circuits/Interface/ir_switch.htm , everything is ok, the 555 works correct, but whatever signal is entered in 7474, the circuit is not working. in my opinion the circuit diagram is not wrong. any ideas ?
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davorin
Joined: 07 Jun 2003
Posts: 1490
Helped: 8
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| 08 Oct 2004 16:42 Re: problem with 7474 bistable |
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1. Do you have 5V supply for the 7474? Asuming yes if you followed the schematic (o;
2. Is the LED blinking when you press your remote control?
3. If so..is there a signal coming out of the 555? And you also see it at pin3 of 7474?
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golfbumb
Joined: 29 Sep 2004
Posts: 40
Helped: 5
Location: California
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| 08 Oct 2004 17:04 Re: problem with 7474 bistable |
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Actually, the schematic appears to be wrong. In the circuit, Preset-not and Clear-not of the 74LS74 are wired low. According to the 74LS74 data sheet, that is an unstable configuration that will forever output logic high on both Q and Qnot, regardless of clock activity. Wire preset and clear to logic high if you want it to operate as a multivibrator. If it still does not work then double check your VCC (pin 14) and ground (pin 7) [for a 14 pin device].
Download http://focus.ti.com/lit/ds/symlink/cd74ac74.pdf for a datasheet. (Fairchild and National Semi datasheets state the same thing.)
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jayxflash
Joined: 28 Nov 2003
Posts: 8
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| 09 Oct 2004 0:00 Re: problem with 7474 bistable |
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| ok guys, that was the problem, those 2 pins had to connect to Vcc not to GND. now it works just fine. thanks for help!
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