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Driving a LED with D type Flip-Flop

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caius:
Hi,
I needed a simple circuit for driving a diode LED with a D Type flip-flop like the 74LS74 so I made my own design.As you can see from circuit the LED should lit with the pressure of siwtch that pulse HIGH the CLOCK input so the negated output will be high and light the LED.It doesn't work well.I got different results using same 74LS74  from different manufacturers.Sometimes the LED will turn on, sometimes not.I tried also different TTL family and circuit works almost fine with a 74ALS74
Am I  wrong in something?

P.S.
The PRESET input come from the output of an OR gate whose inputs are two active LOW signals pulled HIGH by 10K resistors

glarsson:
Yes, you are clamping the inverted output by the LED voltage and it is not guaranteed that this voltage is seen as high by the D input. Put a resistor in series with the LED or use a gate to drive the LED and nothing else.

Ian.M:
Also, all the bipolar TTL families are *much* better at sinking current than sourcing it.  CMOS 74xx devices are more symmetric, but there's still usually an advantage when sinking current.  Therefore move the LED to the Q output and put it + a (mandatory) series resistor between the output and Vcc.

Zero999:

--- Quote from: Ian.M on November 22, 2018, 02:33:31 pm ---Also, all the bipolar TTL families are *much* better at sinking current than sourcing it.  CMOS 74xx devices are more symmetric, but there's still usually an advantage when sinking current.  Therefore move the LED to the Q output and put it + a (mandatory) series resistor between the output and Vcc.

--- End quote ---
I agree. Failing that, drive the LED using a BJT, with a suitable current limiting resistors: one for the base and another for the LED.

caius:
Thanks for all the advices.I already tried to move the LED to the Q output and at same time use a resistor between the output and VCC but the LED is always lit.
I could use a simple gate or a transistor to drive the LED but thne I should change the design (nothing hard though ).As you can see from this other snippet of schematics the Q output is routed to a NOR gate and CLOCK inputs of two Flip-Flop

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