Make a LED blinking with a LOW pulsing TTL signal - Page 1
Forum section: electronics / beginners

author: caius
author_rank: Regular Contributor
timestamp: December 11, 2019, 11:12:31 am
content: Hi all,sorry for dumb question.I have the need to make a LED blinking using a TTL signal (outputted from a 74LS32).The signal is HIGH (5 volt measured) with a LOW pulse (an assertion) of ~2.5us.I tried with and without a transistor, the LED (both normal and high efficiency ones) is blinking but very faintly.Please, let me know if you need more details or measurements.Thanks in advance.P.S.I would like to achieve this result in a simple way without using a microcontroller.

author: macboy
author_rank: Super Contributor
timestamp: December 11, 2019, 02:09:20 pm
content: You need a circuit to stretch the pulse, since 2.5 microseconds is too short to perceive.Here is something I knocked up quickly in LTSpice (a free simulator from Analog Devices/formerly Linear Tech). Here, the input pulse turns on Q1 which quickly charges the capacitor C1. The charged capacitor then turns on Q2 which drives the LED. The resistors R3 and R2 discharge the capacitor and ensures that Q2 turns off the LED when the voltage of C1 drops below a threshold voltage (otherwise the LED would slowly dim down). With this circuit, the LED stays lit for over 100 ms, with an input pulse of 2.5 us.

author: EEEnthusiast
author_rank: Frequent Contributor
timestamp: December 11, 2019, 02:12:35 pm
content: Use a 555 timer as a monoshot.

author: caius
author_rank: Regular Contributor
timestamp: December 11, 2019, 02:53:12 pm
content: Thanks for replies.Obvioulsy the LED should blink as long as the LOW pulse is present then it must stay off.Going into specific the LOW pulse comes from a 74LS32 whose inputs are normally both HIGH then at a certain time one goes fixed LOW and the other stays HIGH but with a LOW pulse of 2.5us.I used an OR gate because the LED must blink only when a certain condition (both input signals LOW) is satisfied.@EEEnthusiastI thought me too about using a NE555 in monstable mode like shown in the attached circuit.

author: Caliaxy
author_rank: Frequent Contributor
timestamp: December 11, 2019, 04:12:56 pm
content: Both solutions should work. Make sure though the stretched pulse is shorter than the repetition rate of the 2.5 us pulse, otherwise you'll miss events. For the RC values in your 555 diagram, the LED stays on for about 2.4 s. In this respect, the discrete solution might be better, as it is retriggerable, i.e. a new event occurring within the stretched pulse will recharge the capacitor and extend the time the LED is on (which might help you notice a repeated event or not). You might as well use a dedicated monostable (such as 74LS121). Whatever you have in your drawers...Alternatively, you can keep your pulse short and increase the current through the LED (reduce the LED series resistor and/or use a higher voltage to power the LED, if available). LEDs can survive currents higher than their rated "continuous forward current" for short periods of time ("peak forward current") and get brighter. Or you can combine both methods. Plenty of optionsHave fun!

author: caius
author_rank: Regular Contributor
timestamp: December 11, 2019, 07:12:23 pm
content: I tried the circuit with the two transistors, LED is always lit (a litte dimmer) although it blinks (light becomes stronger) when the pulse is active.I used a BC547 and a BC557 as equivalent of 2N3904 and 2N3906 since I have no stock of them.This with an high efficient LED, with a normal one it's always lit (dimmer) with no blinking at all.The problem is that on the base of Q1 there is always a positive voltage applied (logic HIGH) which is the output of the 74LS32 when there is no LOW pulse.

author: macboy
author_rank: Super Contributor
timestamp: December 11, 2019, 07:48:00 pm
content: Add a 1k (or less) resistor from base to emitter of Q1. This will definitely help to prevent it from turning on with a weak signal.

author: caius
author_rank: Regular Contributor
timestamp: December 11, 2019, 09:17:18 pm
content: Quote from: macboy on December 11, 2019, 07:48:00 pmThanks, it works fine with a 1K or lower resistance (I tried 470 and 330 Ohm).Now the LED is not always lit and blinks only when there is the pulseNow the hardest part.I want to make a LED blinking when a single pulse of 2.5us (and not a train like in the previous case) is present.I'm thinking abut a flip-fop or a counter to achieve this.Maybe a blinking LED (the ones that blink from themselves, I mean) can be useful too.Anyway, just to point out, the two circuits with the two LEDs are totally independent from each other.