Electronics > Beginners
Driving a LED with D type Flip-Flop
glarsson:
Very true.
Typical resistor values was 100 ohm when driving high to ground and 220 ohm when driving low from Vcc.
Also, when driving from high to ground you had to watch the current through the Vcc pin. Driving six status LEDs using a hex inverter would pull 200mA from Vcc. Not acceptable. Design restrictions like this is one reason why you often find NPN transistors driving LEDs in old TTL designs. Completely different today when LED currents below 1mA is plenty.
caius:
Tried all the advices with no luck (with both normal and high efficiency LEDs).The weird is that one circuit (the one that uses flip-flop 1) do work (LED is ON when switch is pressed and OFF when pressed again) while the other circuit don't /the one of flip-flop 2).I'm using the two flip-flop of a same 74LS74 to save a chip.Perhaps I have to try to use two dedicated chips.I attach schematics (pin numbers are the same but I'm using the two different flip-flop).As you can se the two circuit are identical.
Ian.M:
I'm not surprised it doesn't work. IIL for a LS TTL gate is -0.4mA, and VILis 0.8V. That mean that your switch input circuit has to be able to pull down to below 0.8V to reliably deliver a logic '0', and has to be able to sink 0.4mA while doing so. 2x 10K resistors in series hasn't got a snow-flake's chance in hell on a hot day of doing that, so if you are seeing *any* activity, its purely due to noise pickup!
If any design using LS TTL gates has more than 470R in series with any input, its FUBARed.
glarsson:
For TTL the general rule is to use pull up resistors and connect switches to ground.
TTL is good att pulling down to ground but pretty bad at pulling up. This is also reflected in how the inputs behave. Don't fight it.
caius:
--- Quote from: glarsson on November 22, 2018, 08:10:30 pm ---For TTL the general rule is to use pull up resistors and connect switches to ground.
TTL is good att pulling down to ground but pretty bad at pulling up. This is also reflected in how the inputs behave. Don't fight it.
--- End quote ---
I got better result using pull-up resistor and tie swtiches to GND, thanks.I wonder if the 0.1uF capacitor (or perhaps less capacitance) is still needed as well as the 10K resitors in series with TTL clocK input.
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