Hack 01-01: Stretching the Flash

Tags

LED, flasher, blinker, pulser, oscillator, multivibrator, pulse width, duty cycle.

Difficulty Rating

2 on my scale, if you're starting from here. But if you already built the last project, then this hack is just a 1!

Purpose

A simple improvement to our starting circuit, the LED flasher. This change will make the pulse of the flash longer and seemingly brighter.

Bill of Materials

We need the circuit from Project 01, assembled and working, plus this:

Label	Description	Image
R4	Resistor, 100 kilohms (= 100,000 ohms). Color stripes: brown-black-yellow.

Assembly

Click on the image to see a larger version.

We need to add just one part, R4. But first, move the bottom lead of C1 over 1 row to the right, as shown. Then add R4. We're essentially wiring R4 in series with C1.

Success is when the LED flashes, like before, but with a wider pulse. The change will seem brighter to our eyes, even though we're not driving the LED any "harder".

Schematic

Click on the image to see a larger version.

You should notice here how R4 is wired in series with C1.

Images

Click on the image to see a larger version.

Here are voltage waveforms of nodes n2 thru n6. (Nodes n0 and n1 waveforms are simply flat lines -- not too interesting). These are taken from the circuit simulator software, and not from an oscilloscope connected to a live circuit. So they may be slightly off from reality, but still informative.

How Does It Work

Just looking at the waveforms, you can see that the LED "flash" (pulse) is much wider than before. In our starting circuit, the output pulse was around 0.5 millisecond (1/2 of a thousandth of a second) wide. Now, it's like 40 milliseconds (40 thousandths of a second) wide -- 80 times wider. These fractions of a second may sound too short to matter. But really, a 40 millisecond flash is easily noticeable to our eyes.

In the last article, I went to great detail about how this circuit works. I explained how C1 provides positive feedback, and that shot of electricity that was fed back via C1 gets used up quickly by T1. Now adding R4, it gets used up much slower. We stretched out the feedback pulse. Because the feedback pulse is longer, the LED is "on" for a longer time.

Also, I said in an earlier article how a resistor is useful as a "voltage-to-current adjuster". The resistor wired in series with something reduces the current flow. So R4 also converts the positive feedback pulse from a voltage to a current.

Simple Mods

If we can stretch it out this much, then can we go farther? Sure, give it a try. The circuit may still work if R4 is 220 kilohms, 470 kilohms,... maybe even 620 kilohms. Above some value, the feedback will be too weak and the circuit won't oscillate.

Variable pulse width? Yes, we could replace R4 with a variable resistor. There are many variable choices: a potentiometer (manual adjustment), photocell (light-sensitive), thermistor (temperature-dependent), flexistor (bending/flexing). The variable resistor would convert our circuit into a sensor interface, useful for monitoring some environmental condition. You could even feed the pulses to a computer or smart-device. Software would make the pulse width measurement become meaningful data.

Known Weaknesses and Their Fixes

Battery Life — Before we added R4, the flash of the LED was very brief and seemed dim. But it used less battery-power than with this mod. With R4, the LED is lit for longer amount of time. And the LED is the main consumer of power in the circuit. If you want to maximize battery life so the circuit can operate for many months, your choices are:

1. Keep Pulse Short — Use a lower value for R4, like maybe 47 kilohms (47,000 ohms). Try even lower values to see what you like.
2. Reduce Pulse Current — Maybe you don't need the flash to be so bright. You can increase the value of R3 from 68 ohms to 100 or 150 ohms.
3. Slow Pulse Rate — Less pulses overall means less power used. I discussed how to change this earlier.
4. Bigger Batteries — You could use higher-capacity batteries, like bigger sizes (C or D cells) or chemistries (alkaline, lithium). This last idea is a choice more for the advanced hobbyist.

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