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Friday, May 30, 2008

The oscillator will be used to generate a square wave at a desired frequency. The wave is fed into a transistor that drives an infrared LED on and off very rapidly. Because the emissions are infrared and very fast, neither is visible to the human eye.

Inexpensive infrared receiver chips are available at 36 kHz, 38 kHz, and 40 kHz. The receivers are sensitive to oscillations several kilohertz to either side, although reception distance improves with a better signal to start with.

If used for object detection, the signal needs to travel the distance to the object, bounce off the object, and then travel the distance back to the receiver. So, distance becomes a factor.

Because infrared receivers amplify the signal to improve detection, electrical noise generated from the oscillator can leak into the receiver and trigger a false detection. This isn't a problem for VCRs or most consumer devices as they tend to contain either a transmitter (remote control) or a receiver (CD player), but not both.

Therefore, robot transmitter and receiver circuits must be carefully designed and positioned apart to be useful. Robots that chase electrical ghosts, spin in place, or jerk sporadically are initially amusing, but eventually frustrating.

The lower the power of the circuit, the more likely it will be lower in noise. Also, liberal use of decoupling capacitors and metal shielding helps a lot. Greater distance between the circuits makes an enormous difference.

The Popular 555

The 555 IC is an extremely popular timer. The low-power CMOS versions (TLC555, LMC555, and ICM7555) use less power than the older (555, NE555, LM555) versions and don't require a capacitor on the control pin. Although pin and functionally compatible, the component values differ between the low-power CMOS and older versions.

Infrared Emitter 555 Schematic

A portion of the configuration presented here is similar to an example in the Maxim ICM7555 datasheet. In this circuit, the 555 is used in astable multivibrator mode.

For maximum effect, over 60 milliamps pulses through the infrared LED. Adjust R3 as appropriate for your use and LED specifications.

When calculating current through the resistor, don't forget to first subtract the voltage drop across the LED and transistor. Let's say the LED uses 1.8 V (1.6 V to 2.2 V wouldn't be unheard of). Let's say the collector-emitter drop of the transistor uses 0.2 V.

5 V (total) - 1.8 V (LED) - 0.2 V (transistor) = 3 V remaining to drop across the resistor.

3 V / 47 ohms is about 64 mA. Because there's only one path, the current going through the resistor must be the same as what's going through the LED.

Now for the other trick: the word "pulses". The LED is only on half the time because it is blinking. If you use an ohmmeter, the average current is 32 mA (half).

Aside: The LED heats up faster than it cools off. As such, it's not possible to drive 100 mA through a 50 mA LED even though the average current is half. Depending on ambient temperatures, it's usually safe to drive only 125% or 133% of rated maximum at 50% duty. With smaller duty cycles and frequent pauses, it's possible to drive a lot more current in very short bursts.

Theoretical frequency can be calculated by:

f=1/(1.4 RC)

where f is frequency in kilohertz, R is resistance in kilohms, and C is capacitance in microfarads.

38 kilohertz = 1/(1.4 * 18.7969 kilohms * 0.001 microfarads)

Even if you could find a 18796.9 ohm resistor, it turns out the capacitance and resistance of the wiring and the wide tolerance (even at 1%) of the parts means a variable resistor (potentiometer) is a must! Also, the current being used to drive the transistor (Q1) alters the timing a bit.

Using a 1-nF (C1) [1 nF is same as 0.001 µF] capacitor and a 15-kilohm fixed resistor (R1) plus a 5-kilohm potentiometer (R2) does the trick. Not only does the potentiometer allow for hand tuning, but also the frequency can be varied from about 32 kHz up to about 42 kHz. The margin means the desired values of 36 kHz to 40 kHz should be attainable even with variations in parts and wiring.

Solderless Breadboard

There's a slight change from the official schematic presented above. On the breadboard, the timing capacitor (C1) is connected to +5 V rather than ground. Testing indicates the same frequency, voltage range, and power consumption regardless. Still, you should use a connection to GND.

On the left is a multiturn potentiometer. The small brass-colored screw rotates many times to perform the same adjustment as the white single-turn dial on the right. The multiturn allows for more precise adjustments and is less prone to shift out of position. Even if it does shift, less change results because it needs to take multiple turns around.

Multiturn potentiometers are more expensive, but worth it for timing circuits.

Surface Mount Components:

A very small infrared circuit can be created with surface mount components. The power usage and basic functionality is the same. Theoretically the electrical noise should be reduced since through-hole leads can act as transmitter antennas.

A couple of notes:

* R2 is a very small (4 mm) surface-mount multiturn potentiometer.

* A lot of extra drill holes exist in the board so that I could use up my small stock of through-hole parts if desired (for this board, I didn't desire). For example, C2 has extra holes to the left and right.

* This circuit differs from the breadboard and schematic as it is designed to drive a pair of dual emitters (four LEDs total). A tiny, green, surface-mount LED indicates when the oscillator is enabled.

create a surface-mount boards at home.

It's really not difficult. You should try it!

There are a few tricks that help me...

Using a toothpick, I place a small dab of silicone adhesive (which I suspect is just caulk in a smaller, more expensive tube) on each location of the board where a surface-mount component is going to be.

As far as I can tell, silicone is pretty friendly compared to the usual nasty PCB chemicals. The package indicates the material is stable up to 400 degrees Celsius. The wet viscosity holds components in place but allows for nudging. The thickness is perfect for bridging the space between the board and slightly raised component backs.

I then deposit each component using tweezers. After five minutes or so, the components can be soldered without them moving around. If a mistake is discovered, the component can be pulled off easily, as the silicone dries to a pliable, soft, rubbery consistency.

With small components, sometimes solder bridges form and sometimes the core flux misses the mark and cold solder balls or joints form. No problem, just place some flux paste in those locations. Reheat the joints with the tip of the soldering iron and they melt to perfection.

Which Oscillator Is Better?

The 555 circuit uses less power when turned on, but the total power usage of an infrared emitter device easily overshadows any minor power-on savings in the oscillator itself.

The NAND solution is cheaper and basically as easy to construct.

Obviously my optimized NAND circuit is better than my original NAND circuit. However, I can't say whether my 555 circuit is even better.

Since the 555 is specifically designed for timing and since it has been widely adopted, I assume there are benefits that I haven't perceived. Most likely the ability to vary duty cycle and other versatility is the real reason the 555 is so popular.

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