Tags
LED, flasher, blinker, pulser, oscillator, multivibrator, variable frequency, voltage control.
Difficulty Rating
2 on my scale.
Purpose
A simple improvement to our recent mod to the LED flasher. This change will allow us to manually adjust the pulse rate (frequency).
Bill of Materials
We need the circuit from last article, assembled and working, plus this:
| Label | Description | Image |
|---|---|---|
| POTM | Potentiometer, 100 kilohms (= 100,000 ohms), linear taper. |  |
Your potentiometer should have some wire leads soldered to the terminals, so these wires can be plugged into the breadboard holes. Or, you can use clip leads to connect it to the circuit.
Assembly
Click on the image to see a larger version.
We need to add just one part, POTM. But first, move the bottom lead of R1 over to an unused row to the right, as shown. Then add POTM by connecting its 3 leads as shown.
Success is when the LED flashes, and the flash rate changes as you adjust the potentiometer.
The flashing should stop and the LED dark when the potentiometer adjustment is turned fully counter-clockwise (CCW).
Schematic
Click on the image to see a larger version.
The POTM (the potentiometer) is wired as a variable voltage source; its output is the wiper terminal (POTM's middle terminal). Resistor R1 is rewired so we now have a voltage-control input to the flasher circuit.
Images
Here are voltage waveforms of nodes n2 thru n6 from the simulator software. (Nodes n0 and n1 waveforms are simply flat lines -- not too interesting).
I have 3 sets of waveforms. Each one has the POTM set at a different position.
With POTM set at just 30% of full rotation (almost fully counter-clockwise), the flash rate is slow:
Click on the image to see a larger version.
Next, with POTM set to 50% (mid-rotation), the flash rate gets faster:
Click on the image to see a larger version.
And finally, with POTM set to 100% or fully clockwise, the flash rate is at its fastest:
Click on the image to see a larger version.
How Does It Work
What is a Potentiometer?
A potentiometer has 3 terminals. The outer two terminals — let's call them terminals #1 and #3 — are the two ends of fixed resistor. In this hack, I specified a 100 kilohm (100,000 ohms) potentiometer in the Bill of Materials. That means the potentiometer resistance between terminals #1 and #3 is 100 kilohms. This resistance does not change even if you turn the potentiometer shaft.
This resistive element is unsealed and exposed, unlike a normal resistor. It is made this way so a sliding metal contact can be moved along its surface.
Terminal #2, the middle terminal, is connected to this wiper. The wiper is an internal contact that moves from one end of the resistive element to the other.
You could make your own crude potentiometer with pencil, paper, and 3 clipleads. Pencil lead contains graphite, a form of carbon, which is used in actual resistors and potentiometers. Draw a continuous line on paper with the pencil. You just made a resistor. The resistance value (in ohms) will vary depending how wide, how long, and how heavy you draw the line. Attach a clip lead to each end of the line. Use the third cliplead as the wiper: hold it in your hand, touch it to the pencil-line, and manually slide it across the line, from one end to the other.
Potentiomers are typically made in values from 100 ohms to 1 megohm, and not in as many value choices as actual resistors. The most common values are 10 kilohms, 50 kilohms and 100 kilohms.
The available sizes is a very wide range, from micro-miniature, to large high-power wirewound types. The most common is the panel-mount carbon-composition type like we are using here. This type is meant for lower-power or signal adjustments.
This Circuit Mod
As I mentioned in an earlier article, a resistor is useful as a "voltage-to-current adjuster". A resistor is used to reduce current flow. A resistor can convert a voltage source into a crude current source. It can work well when the resistance is much higher than the node resistance that it's sourcing.
In this circuit, resistor R1 converts a voltage into a current that flows into node n3. The voltage source in the old circuit was node n2, but in this version, it is the POTM wiper. So when we adjust the voltage by turning POTM, we're also adjusting the input current. This current controls the "off" time of the LED. A higher input voltage sets a higher input current, which sets a shorter "off" time.
Simple Mods
Widen the Adjustment Range — The range that you can adjust the flash rate is somewhat limited, from about 1 to 4 flashes per second. To increase the range, try these changes, alone or together:
- Decrease R1 — The resistance R1 can be decreased. Try 2.2 megohms (2,200,000 ohms), or 2.0 megohms. If you reduce it too much, below 2.0 megohms, the circuit will stop oscillating when POTM is at 100% rotation — the LED will stay lit — but maybe you'll like this new "feature".
- Reduce R4 — Making this resistor a lower resistance, like 10 kilohms (10,000 ohms) will shorten the LED "on" time but will allow the circuit to run faster.
The above changes, done together, may squeeze out a little more range, like from 1 to 5 flashes per second. I think that's about as good as you'll get with this simple circuit.
Known Weaknesses and Their Fixes
This mod is good for learning things like the potentiometer, making a circuit adjustable, and voltage control. But it does have limitations.
- Adjustment Range — I haven't been able to get more than about 5-to-1 adjustment range in the flash rate, even with changing the other resistor values. (Hint: try higher battery voltage). Maybe you will have more luck. I think if you want a wider range, I'd look at completely different circuit design. Maybe I'll cover this in a future article.
- Full Use of Potentiometer's Range — I mentioned how the circuit only oscillates if POTM is at least around 30% rotation. Below that, it shuts off. So the bottom 30% of POTM adjustment is wasted. We can remedy this by inserting a resistor, like 33 kilohms (33,000 ohms), between terminal #1 and node n0. This addition raises the voltage at minimum adjustment so it's about equal to the minimum oscillation point.
| ← Previous Article | ↑ | Next Article → |
| Home Page |
No comments:
Post a Comment