tt1234 Posted July 8, 2005 Report Share Posted July 8, 2005 Hi, I need help to get a one-shot pulse generator withboth positive and negative trigger input. 555 timeronly takes negative trigger input. 74LS123 can takeeither positive or negative but not both. Thanks a lot for help! Tom Quote Link to comment Share on other sites More sharing options...
NSklavos Posted July 11, 2005 Report Share Posted July 11, 2005 I have used the following attached schematic using a 7486 exclusive or (XOR) chip. It is basically a digital frequency doubler. It relies on the propagation delay of the second and third section to provide an output pulse. With Pin 1 High, the output (pin 11) will be High with a negative going pulse. With Pin 1 Low, the output (pin 11) will be Low with a positive going pulse. Pin 2 is the clock input or a hi/lo switched pulse. The output pulse is pretty narrow so you may want to follow it with a 555 timer or one shot chip to get a desired pulse width out of it. You can probably use a cmos chip like a 4070 for the XOR also, I believe just the pinouts are different. Quote Link to comment Share on other sites More sharing options...
heathtech Posted July 13, 2005 Report Share Posted July 13, 2005 I have an idea for the 555 timer that might work. Please someone correct me if this is wrong. I don't want to post a mistake, but I wanted to figure it out and have someone correct me if I'm wrong: Quote Link to comment Share on other sites More sharing options...
audioguru Posted July 13, 2005 Report Share Posted July 13, 2005 Hi Heathtech,Your circuit will work if the trigger inputs have a duration that is less than the timing period.The OR gate followed by an inverter can be replaced by a NOR gate. ;D Quote Link to comment Share on other sites More sharing options...
Guest Alun Posted July 14, 2005 Report Share Posted July 14, 2005 Try this circuit, connect the output to pin 2 of the 555. Quote Link to comment Share on other sites More sharing options...
xoy Posted July 14, 2005 Report Share Posted July 14, 2005 hi anyone!!!can someone help me pls...my problem is this..i want to time a counter, so that it will automaticlly switch-off after one minute or 60 seconds.will you pls give me simple schematic or circuit digram of 60 seconds timed switch-off.any reaction regarding to this are highly appreciated... tnxx!!!!!___xoy____ Quote Link to comment Share on other sites More sharing options...
audioguru Posted July 14, 2005 Report Share Posted July 14, 2005 Hi Xoy,Set the 555 as an oscillator to have a full cycle in 6 seconds. At the end of each cycle, it latches the counter's reading and resets the counter. Then add a zero display digit to this display's right side and the whole display shows pulses per minute, updated each 6 seconds. ;D Quote Link to comment Share on other sites More sharing options...
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