Electronics > Beginners
Make a LED blinking with a LOW pulsing TTL signal
caius:
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.
macboy:
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.
EEEnthusiast:
Use a 555 timer as a monoshot.
caius:
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.
@EEEnthusiast
I thought me too about using a NE555 in monstable mode like shown in the attached circuit.
Caliaxy:
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 options :)
Have fun!
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