| Electronics > Beginners |
| Driving a LED with D type Flip-Flop |
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| glarsson:
A 10k resistor is an alien creature in a TTL circuit. You will not be able to pull down a TTL input through a 10k resistor. |
| Ian.M:
You *cant* use a 10K resistor in series with a bipolar TTL input. It just wont work - see my reply #12 above. If you need a clean clock to a flipflop from a button and you are using LS TTL, there aren't many good options. Its very hard to get acceptable results from RC debouncing as the maximum practical limit for the series resistor is under 2K, so you need a *BIG* capacitor and a gate with Schmitt trigger inputs. You can use a retriggerable monostable. However the classic way to implement a debounced switch with LS TTL is to use a SR-flipflop with active low inputs, and a SPDT switch, common to ground, and the two ways to the two flipflop inputs, with pullup resistors on each. The result is an output that cant toggle until the switch moving contact has moved from one fixed contact, all the way to the other. For the SR-flipflop, you can use /SET and /CLR on a 74LS74 with D and CLK grounded, or a cross-coupled pair of NOR gates for the classic active low implementation. Don't use NAND gates as if the SR inputs are active high, and the switch grounds the inputs, it will enter an indeterminate state during contact bounce and transition. |
| caius:
--- Quote from: glarsson on November 22, 2018, 09:43:07 pm ---A 10k resistor is an alien creature in a TTL circuit. You will not be able to pull down a TTL input through a 10k resistor. --- End quote --- Yes, I'm aware of this, they are used only as pull-down/up.But with this 10K resistor in series, the circuit made with flip-flop 1 (and switch tied to VCC) is working.Maybe only luck.... :) Besides, is the resistor between LED cathode and GND needed?I guess a 100 ohm could be fine. |
| David Hess:
There are two options which work well for TTL: 1. The preferred solution is to pull the LED cathode to ground with the TTL output and include a series resistance of like 470 ohms for a roughly 4 milliamp LED current. 2. If the LED must be driven with a TTL high output, then a pull-up resistor to the TTL output is going to be needed. By itself, a TTL output cannot source enough current to drive an LED well. 4.7 kilohms might be a typical TTL pull-up resistor value but 1 kilohm will be needed to get even 2 milliamps through the LED. |
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