Problems with IR Tachometer

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philba
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Re: Problems with IR Tachometer

Post by philba » Sat Aug 06, 2005 6:33 pm

I think the 14553 has internal latches. The latch signal causes the current count to be latched. The bcd outputs appear to be driven from the latches, not the counters. It appears that the display gets updated every 10 seconds. This can be improved to 7 seconds by using 6 to latch (pin 5 instead of 6) and then use 7 (pin 6) to reset the 14553 and the 4017. Still, 7 seconds seems like an awefully long update time. <p>You might be able cut a second of the time by driving the reset off of the 6 digit, using the nor gates to pick a signal off of one of the 4040s that gives enough delay to allow the latch operation to complete.

rshayes
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Re: Problems with IR Tachometer

Post by rshayes » Mon Aug 08, 2005 5:30 am

Unfortunately, the 6 second update is necessary to make the display read in 10s of RPM. This could be reduced to 3 seconds by counting both edges of the input signal, but this would be substantially more complicated.<p>The latch appears to be edge triggered. There is a note on the Motorola data sheet that indicates that "Information can be recovered from the latches after the counters have been reset if Latch Enable remains high during the entire reset cycle."<p>If states 0, 1, 2, 3, 4, and 5 are used for counting, then state 5 can be inverted and used for Latch Enable. This would latch the output at the end of state 5. Latch Enable would remain high except during state 5. Since Latch Enable is edge sensitive it should be possible to eliminate C6 and R20.<p>State 9 can be used for Reset. Since Latch Enable is high during this period, it appears that the display would remain undisturbed.<p>In this case, the counter would be held at zero until the beginning of state 0. Latch Enable would have a positive edge at the end of state 5, and the counter contents would be transferred to the display at this time. If the latches are edge triggered then further counts would not be displayed.<p>This would eliminate the need for a external gate, which would allow IC7 and IC8 to be deleted. One of the unused sections of IC6 could be used for the inverter.<p>This would display a number continuously, with the count being updated every 10 seconds.<p>Capacitor C7 could be eliminated by using one of the outputs of IC5 to drive pin 4 of IC2.

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philba
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Re: Problems with IR Tachometer

Post by philba » Mon Aug 08, 2005 11:37 am

good point on the inverted input. <p>the schematic seems to have dissapeared so its hard from memory but wouldn't using the falling edge of the pulse mean you are sampling for 5.5 seconds?

Rubi
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Re: Problems with IR Tachometer

Post by Rubi » Tue Aug 09, 2005 12:34 am

Hi Philba,Stephen<p>The Schematic has not dissapeared, Bill has corrected it.
After a first look, he only corrected the error of driving the Display Transistors.<p>Cheers
Michael

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Michael Kaudze
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Re: Problems with IR Tachometer

Post by Michael Kaudze » Tue Aug 09, 2005 9:30 am

New Schematic is posted on SERVO downloads..
Intelligence is the ability to adapt to change. Stephen William Hawking.

grant fair
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Re: Problems with IR Tachometer

Post by grant fair » Tue Sep 20, 2005 9:39 am

Michael-<p>Did you get the tachometer circuit working?<p>Grant
Grant

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