# LED Timing Circuit with Single Gate

#### brightnight1

Joined Jan 13, 2018
91
I'm looking to build a low power circuit with a single gate logic chip to pulse an LED at a certain rate (10Hz) using some resistors, a capacitor, and a diode.I could use a 555 timer but for anyone wondering, I want to do it this way. Is there a name for this type of circuit so that I can look up a reference design? Someone told me I could use the 74LVC1G14SE-7 but I don't understand how to design the circuit:

74LVC1G14SE-7:
https://www.digikey.com/en/products/detail/diodes-incorporated/74LVC1G14SE-7/3829442

#### Sensacell

Joined Jun 19, 2012
3,331
The link below is for the ancient CD4093, it shows the concept.

https://e2e.ti.com/support/logic/f/151/t/91055

A CMOS CD4093 contains 4 gates that can make 4 separate oscillators.

The key is the Schmitt trigger input, the 74LVC1G14SE is a single-channel inverter with Schmitt trigger input, so the concept is the same.

#### Delta Prime

Joined Nov 15, 2019
1,311
Hello there

Is there a name for this type of circuit so that I can look up a reference design
Here you go.

All you need now is a LED along with its current limiting resistor.You are going to run into a lot of math that looks difficult but it's not, replace the resistor with a 1K ohm potentiometer and the capacitor start off with a one microfarad polarized capacitor with the working DC voltage of 10 volts or higher with the power supply 5 volts.
I hope this helps you.

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#### Ian0

Joined Aug 7, 2020
8,954
Is there a name for this type of circuit so that I can look up a reference design?
Yes. It’s called a Relaxation Oscillator.

The Hc14/4093 works exactly the same as the 555 circuit.
The 555 will be much more accurate in frequency.

#### Tonyr1084

Joined Sep 24, 2015
7,562
While generally I'm opposed to solving or designing circuits for others - in part because I'm not that good at it - here's one possible solution. This should give you nearly 10Hz (10 times per second) flash rate. You can adjust VR1 to control the ON time versus the OFF time. Others present on this site will definitely tweak my design or point out errors in my circuit. That's totally welcomed. Depending on the voltage used (5V or higher) and the LED Forward Voltage (Vf) you will need to calculate for the proper resistance. It may look difficult but it's actually fairly straight forward.

#### Delta Prime

Joined Nov 15, 2019
1,311
While generally I'm opposed to solving or designing circuits for others - in part because I'm not that good at it
Your LEDs backwards... Just kidding you're better than most!

#### Papabravo

Joined Feb 24, 2006
20,621
I'm looking to build a low power circuit with a single gate logic chip to pulse an LED at a certain rate (10Hz) using some resistors, a capacitor, and a diode.I could use a 555 timer but for anyone wondering, I want to do it this way. Is there a name for this type of circuit so that I can look up a reference design? Someone told me I could use the 74LVC1G14SE-7 but I don't understand how to design the circuit:

74LVC1G14SE-7:
https://www.digikey.com/en/products/detail/diodes-incorporated/74LVC1G14SE-7/3829442
It is called a relaxation oscillator.
OOPS -- missed post by @Ian0
You can also use an opamp or a comparator. You get better results with a better capacitor X7R or C0G (NP0)

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#### Tonyr1084

Joined Sep 24, 2015
7,562

#### Delta Prime

Joined Nov 15, 2019
1,311

#### brightnight1

Joined Jan 13, 2018
91
Thanks for all the replies, I was able to simulate and test a Schmitt trigger oscillator in LTspice which was awesome! Now I realize it isn't exactly what I wanted, or I'm not sure how to make it what I want. I mentioned in my first post that I wanted to blink an LED at 10Hz...

What I should've mentioned is that I want to flash an LED at 10Hz but I only want to turn the LED on for a 1us pulse. I was playing around with the resistor and capacitor values in my trigger and not sure how to accomplish that. If I want a false pulse, that means I need a small cap value so it charges and discharges quickly. If it charges quickly then it will triger again quickly so not sure how to solve that problem.

Can I pick values for a schmitt trigger oscillator design that allow me to blink an LED for 1us at 10Hz with this type of circuit?

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#### jpanhalt

Joined Jan 18, 2008
11,087

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#### Audioguru again

Joined Oct 21, 2019
6,461
A 1us LED pulse might be so quick and dim that you might not see it. I read that 30ms looks as bright as an LED at the same continuous current but shorter durations look dimmed like PWM dims it.

#### brightnight1

Joined Jan 13, 2018
91
A 1us LED pulse might be so quick and dim that you might not see it. I read that 30ms looks as bright as an LED at the same continuous current but shorter durations look dimmed like PWM dims it.

#### AnalogKid

Joined Aug 1, 2013
10,796
A 1 us pulse at a 10 Hz rate is a 100,000-to-1 duty cycle. For a simple relaxation oscillator with 1 capacitor, this is not a practical design; the two resistors would be something like 1 K and 100 M. Better to separate things into two stages, a 10 Hz squarewave oscillator and a 1 us monostable. This can be achieved with two 74xxx14 gates, two of the NAND gates in a CD4093, or two of just about anything else that has predictable hysteresis.

Because the output pulse is so much shorter than the oscillator period, the 1 us pulse former does nto have to be a true 2-gate monostable with feedback. A simple differentiator (boxcar) circuit will work fine. What is the output current required?

ak

#### MisterBill2

Joined Jan 23, 2018
16,630
Hello there

Here you go.
View attachment 222546
All you need now is a LED along with its current limiting resistor.You are going to run into a lot of math that looks difficult but it's not, replace the resistor with a 1K ohm potentiometer and the capacitor start off with a one microfarad polarized capacitor with the working DC voltage of 10 volts or higher with the power supply 5 volts.
I hope this helps you.

You can use the circuit shown, but use a CD4049 hex inverter, and put the other 5 inverters in parallel to drive the LED. even at 5 volts that will provide enough current to light the LED very brightly, it the inverters are used to pull down the negative side of tyhe LED. the 4049 can sink much more current than it can source.

#### brightnight1

Joined Jan 13, 2018
91
You can use the circuit shown, but use a CD4049 hex inverter, and put the other 5 inverters in parallel to drive the LED. even at 5 volts that will provide enough current to light the LED very brightly, it the inverters are used to pull down the negative side of tyhe LED. the 4049 can sink much more current than it can source.

Awesome, thank you all for the advice I appreciate it!

#### AnalogKid

Joined Aug 1, 2013
10,796
You can use the circuit shown, but use a CD4049 hex inverter,
Sure about that? The 4049 does not have hysteresis. Try the CD40106.

And, as above, that circuit cannot produce a 1 us / 100 ms duty cycle.

ak

#### Danko

Joined Nov 22, 2017
1,775
What I should've mentioned is that I want to flash an LED at 10Hz but I only want to turn the LED on for a 1us pulse.
Current of Ic(Q2) and LED U1 can be adjusted up to 180 mA by resistor R2.
Current, consumed from source 5V is about 4.5uA.

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