Page 3

Semiconductor devices used in BEAM Robotics
This page has been adapted from Solarbotics Library

P1   P2      P4   P5   P6   P7   P8   P9   P10

Since BEAMbots don't have very high current requirements, diodes only have to be switching devices for low currents. They only pass a signal (current) in one direction (the direction of the arrow).  

1N914 (the same as 1N4148)
This a small-signal diode. You will use it for most of your circuits since they are small and work for logic, or loads up to 200mA. The voltage drop across a silicon diode is 0.7v 

This is a rectifier diode, and doesn't show up as often in BEAMbots, since its only advantage over the 1N914 / 1N4148 is in current capability -- up to 1A. BEAMbots never draw nearly this much current, but remember this diode is used for loads over 200mA. The voltage drop across a power diode is 0.7v and this is constant for any current flow. 

Small-signal Germanium diode -- sometimes salvageable from a variety of circuit boards. In some instances, they are marked with colour bands. The voltage drop across a germanium diode is 0.3v
They are often used in places where the 0.3v drop is an advantage. They are much more expensive than silicon diodes. 

LEDs (Light Emitting Diodes)
These are generally used to show if a circuit is working and will produce illumination with as little as 1mA flowing. 
They can also be used to "drop-a-voltage."  The voltage drop across a LED is 1.7v for red, and approx 1.9v to 2.1v for green and orange. This is called the "characteristic voltage drop" and does not alter for a particular device. However the voltage is slightly different for standard LEDs and high-bright LEDs and depends on the way the LEDs are manufactured. LEDs must always have a resistor in series with them to limit the current. Otherwise they will glow very bright and "blow-up." LEDs will "glow" for 50 years if not overdriven. 
Current-flow does not depend on size. It depends on the efficiency of the crystal inside the body. High efficiency crystals will glow brighter. 
Least current for operation is 1mA. Good brightness is achieved at 15 - 20mA.  Max current is 25mA. 
LEDs come in quite a variety of sizes and shapes (round, square, rectangular, triangular, 3mm, 5mm and jumbo) You can also get infrared LEDs. 
LEDs do not detect light. Use a photo-resistor (LDR- Light Dependent Resistor), photodiode or phototransistor to detect light. Some flashing LEDs seem to have different resistance characteristics when exposed to light as discussed in this FRED article by Ben Hitchcock. 

Flashing LEDs. Flashing LEDs have a chip inside the body to make them flash. They do not need a resistor in series if the supply is between 3v and 9v. 

Most photodiodes can be used for BEAMbots. Some have a wide "field of view" You will need to build them "blinders" so they only respond to light from a given direction.

Phototransistors. Phototransistors are about 100 times more sensitive to light than photodiodes (or LDR's). You can replace a photodiode with a photo transistor in a circuit providing you get the biasing correct. By this we mean the voltage across the device must fluctuate the right amount when different light intensities are detected. 
You cannot simply change a photo diode for a photo transistor without checking the operation of the circuit. Different biasing resistors (and Loads) will need to be added to the circuit. 

Bipolar transistors. These are normal transistors - also called BJT's (Bi-polar Junction Transistors).
BEAMbots use small signal transistors: 
PNP -- 2N3906 (see datasheet for details), BC327-25
-- 2N3904 (see datasheet for details), BC337 
If you have a more-demanding design (i.e., you need higher maximum current), use these:  
PNP -- 2N2907 (see datasheet for details)  
NPN -- 2N2222 (see datasheet for details). 2N2222A / 2N2907 (also PN2222 / PN2907) transistors will  handle 5 to 10 times more current than  2N3904 / 2N3906. 

Field Effect Transistors (FETs) -- JFETs, MOSFETs

Zetex ZRVN2106 -- see the datasheet for details. 
Some of these "new-types" of transistors have a collector-emitter voltage drop of 0.1v or less. This allows them to handle much higher currents in a T0-92 package and remain cool. 

Transistor packaging
T0-92 is the most-common transistor package. Unfortunately, there are multiple pin-out configurations. For bipolar transistors, two are the most common, depending on which type of transistor is available locally (BC* transistors are generally used in Europe and Australia, 2N* transistors are used everywhere else):


   TBR TBR 1381 Voltage Regulator Chip(image courtesy of Solarbotics)
The "1381" is a CMOS voltage-controlled trigger. It comes in a "detection-voltage" from 2v to 4.6v, as shown in the table below.  It detects a voltage on pin 2 and this voltage is then passed to pin 1. 
The following table shows the detection-voltage or "trip voltage" for each device. For instance, a 1381C will trigger between 2.0 and 2.2V.

Rank (suffix)
Trip voltage (V)

Generally, a 1381 is used to build a very efficient solar engine. It determines when the solar engine will "fire." A high trigger voltage will lead to more-energetic activation, but at a less-frequent rate. The 1381J is the most commonly used, although of course, you can pick something different according to your needs. You can also use a diode in series with the earth lead of a 1381 to raise its trip voltage by 0.7v. See the data sheet for details on this chip.
If you can't find a 1381 locally, you may find its European cousin, the TC-54 see  data sheet.


78M0x pinout
78M0x (T0-220) image
5v Regulator   See  data sheet.
The 78M05 is one of a series of voltage regulator chips; this particular chip is popular among BEAMers since it takes a  7v - 35v input, and outputs a nice, steady 5v. This is very useful if you have a 'bot powered by a 9V battery (TTL must be supplied with exactly 5v. CMOS can run on 12v or down to as low as 2v). The 78M05 regulates output voltage to within 200mV (it outputs 4.8 - 5.2v). Unless you get a surface-mount or L version of this regulator, you will be dealing with the T0-220 package (see diagram above for pinout).
Note that to get the most-reliable and most-efficient performance from this regulator, you will need to put filter capacitors across the input and output leads.


(image courtesy of Solarbotics)
Hex Inverter (6-gate inverter chip).
See data sheet (one for 74AC04 and 74HC04)
The 74*04 is a package of 6 individually-accessible inverters (note that these are not Schmitt inverters and as such have no hysteresis built into their response). As a result, this chip is ideal for building a suspended bicore. As it only outputs relatively low amounts of current, any *cores built with a 74*04 will require additional logic "downstream" to amplify the current to levels sufficient to drive a motor.
See the 74*14, below (which is essentially the same chip, but with Schmitt inverters). See also the 74*240, below (which is very similar to a 74*04, but with enables on outputs, and higher output current capability).


74C14 Hex Schmitt Trigger
(image courtesy of Solarbotics)
Hex Inverting Schmitt Trigger (6-gate inverter chip with hysteresis).
See  data sheet.
This chip is considered the heart of Nv net technology, since each Schmitt inverter is individually-accessible (and so each, along with a capacitor, can be turned into a Nv neuron).
See the 74*04, above (which is the same chip, but with "smooth" inverters, rather than Schmitt inverters). See also the 74*240, below (which is very similar to a 74*04, but with enables on outputs).
Note that Schmitt triggers can't easily be used in suspended bicore implementations (so save your '14s for non-suspended designs). As for types of 74*14 chips, a 74HCT14 Nv net has a longer pulse duration (given the same resistors and capacitors) than a Nv net built on a 74AC14 or 74HC14. Meanwhile, for ultra-low-power 5v - 12v applications for Nv, Nu, or oscillator circuits, use a 74C14.


(image courtesy of Solarbotics)
Dual 1-of-4 Decoder / Demultiplexer
See data sheet.
This chip was designed for demultiplexing data signals, but we use its buffers as current amplifiers. The 74*139 is good for turning into motor drivers.


Octal Line Driver
(image courtesy of Solarbotics)
Octal Buffer / Line Driver with Tri-state Outputs
See data sheet.
The '240 is often called "the bicore chip," because we can take advantage of the 240's inverters to turn a single 74*240 into a bicore (in this case, only 2 of the inverters are used, and the rest are used for upping the current so you can drive a motor directly). The '240 also has tri-state outputs, so an enable line can be used to turn its outputs on and off (good for adding reversing capability to a 'bot).
Since the '240 gives us "smooth" (non-Schmitt) inverters, it is usable for either grounded or suspended bicore designs (but better for suspended). See also the 74*04, above (which is very similar to the 74*240, but without tri-state outputs, and with about half the output current capabilities), and the 74*244, below (which is basically a '240 with non-inverting buffers).
Note that pins 1 and 19 are the Enable (tri-state control) pins. Signals on these pins pass through an inverter before getting to the buffer's tri-state control terminals, which means that a LOW at 1 & 19 would be a high voltage at the buffer, so the buffer would be enabled. A HIGH on those pins will produce a low at buffer's tri-state controls and will turn the buffers off. So tie pins 1 and 19 to ground to enable all of the inverters inside the '240 chip.

Words of wisdom from Wilf:
74HC240 and similar HC buffers  Vcc = 2v - 7v but will operate down to 1.0v or less, input voltage trigger / switching level about 1/2 Vcc, supply current when one input is at switching level = 50 mA (power hungry during slow switching), 30 ohm resistance in series with output with Vcc = 5v. Drives small motors. Can give very long bicore time constants if components are carefully matched. Recommended for power smart-head / monocore circuits since time constants remain relatively constant with change in Vcc.

- same as 74HC240 but with "fixed" input switching point of about 1.6v. Faster bicore frequency with same components as HC circuit.

and all other AC devices - same as HC but about 10 ohm series output resistance. Three times drive current of HC, draws over 100mA power supply with input at switch point. Prone to oscillation. No problems / low power when used with fast changing digital input signals and highest efficiency for motor (100mA) driver applications.

74ACT240 - same as AC but fixed input switching level.

A 74HCT240 Nv net has a shorter pulse duration (given the same resistors and capacitors) than an Nv net built on a 74AC240 or 74HC240.


(image courtesy of Solarbotics)
Octal Buffer / Line Driver with Tri-state Outputs
See data sheet.
The '244 provides 8 (thus the "octal") buffers, enableable in banks of 4. This is a very useful chip for amplifying small currents (and so, it is often used to produce motor drivers). Since the '244 has 8 buffers (i.e., 8 current "amplifiers"), it can drive up to 4 motors in 2 directions each, or you can "piggy-back" inputs and outputs to drive fewer motors at higher current.

See also the 74*14, above (which is very similar to the 74*244, but with inverting Schmitt triggers), the 74*240, above (which is basically a '244 with inverting buffers), and the 74*245, below (which is much like the '244, but allows for amplification in both directions).

Note that pins 1 and 19 are the enable (tri-state control) pins. Signals on these pins pass through an inverter before getting to the buffer's tri-state control terminals, which means that a low voltage at 1 & 19 would be a high voltage at the buffer, so the buffer would be on. A high voltage on those pins would be a low voltage at buffer's tri-state controls and would turn the buffers off. So tie pins 1 and 19 to ground to enable all of the inverters inside the '240 chip.


(image courtesy of Solarbotics)
Octal Tri-state Bus Transceiver
See data sheet.
The '245 is an octal buffer chip, and so has 8 channels of buffering power available for our use. This chip was designed for data transmission uses, but we'll use it as a motor driver chip (each buffer essentially amplifies current ) to drive motors from Nv net signals (80mA per channel). Since the '245 has 8 buffers (i.e., 8 current "amplifiers"), it can drive up to 4 motors in 2 directions each, or you can "buddy up" inputs and outputs to drive fewer motors at higher current.

If you need even more current, you can stack '245s, like so: TBR

If you plan on "pushing" your stacked chips' current outputs close to their limits, you should attempt to get an "air-gap" between the chips (put a thin sliver of cardboard between the chips when you solder them together).
For maximum drive current output, you'll need to use the AC / ACT types of this chip (use ACT only if you have well-regulated 5v power available in your circuit). Note that if you need more than about 200mA per motor, you'll need to use an H-bridge, or some similar motor driver, in conjunction with (or instead of) this chip. Let's compare output current capabilities:

  • 74HC245 -- 35mA per output; maximum of 75mA per chip
  • 74AC245 -- 50mA per output; no maximum per chip

The *245 is good for 2-motor walkers. If you want to build a walker with more motors, or one capable of reversing, some designers prefer to use the 74*244 instead.

Miscellaneous guidelines from Wilf:

  1. 74HC/HCTxx non-buffers (74HC14 or 74HC04) draw about half the current consumption, and have about half the drive current compared to HC / HCT buffer chips (74HC240 or 74HC245). Non-buffer chips are thus better for oscillators, say Nv and Nu applications; they are not suited for use in driving motors.
  2. For 4000 or 74Cxx CMOS (i.e. 4069 or 74C14), Vdd= 3v to 12v. These are used for low power oscillators, bicores, voltage comparators, and threshold detectors at 5v. At 3v, these can be used for similar applications with low drive current. CD4049 and CD4050 can drive small loads similar to 74HC/HCT non buffered parts.
  3. For ultra low power applications (low light solar bots) use discrete components or 4000 CMOS combined with the high output current capability of discrete MOSFET H-bridge or 74AC / ACT motor drivers for best overall efficiency.
  4. In general, power consumption and drive vary directly with Vcc; save TTL-level logic for problems that absolutely demand it. For an "average" Nv net, the current at 3v supply is 10x less and at 2v it is 100x less compared to the same chip at 5v.
  5. The ideal BEAM circuit would use a low (2v - 3v) voltage core and sensors combined with level shifting high (5v - 6v) motor drivers to maximize efficiency.
  6. For motor driver applications connected to heavy loads such as motors or coils, use 74ACxxx for increased current capability. For most Nv / Nu applications with outputs connected to LEDs, other chip inputs or to motor drivers, 74HCxxx is recommended. The flip-side of this is that 74ACxxx used in typical BEAM applications uses 4x more supply current than does 74HC/HCTxxx.
  7. 74HC/ACxxx have input trigger levels that are symmetrical and near the midpoint of the power supply.
  8. 74HCT/ACTxxx have trigger levels that are not symmetrical (except near about 3v) and do not track the supply voltage.
  9. 74HC/AC240, 244, 540, 544 chips (and numerous others) can all be used as motor drivers in exactly the same way, observing the differences in pinouts of course. In addition the 74HC/AC241/541 octal buffer has complementary tri-state enable pins (1 and 19 - one active high and one active low), which can save an inverter in some reverser applications.
  10. The 74HCT/ACT bicore has a more predictable time constant and is not as sensitive to motor feedback (if that's desirable). For all other Nv / Nu applications the HC/AC version is generally preferred.

As a general rule, you will want to stay away from TTL-level logic (74*TXXX chips) unless you are prepared to deal with well-regulated 5v power supplies.

AC logic is much faster that HC, has higher gain, and most members of that family (except 74AC14/132) will emit bursts of high frequency oscillation and draw high supply current pulses (>100mA) when operated in quasi-linear (especially Nu) BEAM circuits. At times this causes some difficult to understand or strange symptoms. 74AC is best suited for motor driver applications with all inputs driven rail to rail. Even if used for digital applications, spurious noise and oscillation must be controlled with good circuit layout, signal / supply line routing and supply pin filtering. This has be known to digital circuit designer for a long time and you never see all 74ACxxx designs. Chip manufacturers have come up with newer advanced BI-CMOS logic families which control switching transients but are generally not suitable for BEAM applications.

Solar cells
The quality of the solar collector is one of the most important items in the design of anything  solar-powered. 
As you will see from the table below, the output current from a solar cell varies enormously with different types. This is due to the quality of manufacture as it is very difficult to get all the  voltage-generating pieces to align in the forward direction. Many of the cheap cells are made from "pieces." There is a lot of "bucking" and failing between the wafers in the cell and this limits the production of current. 
Since we have gone to the maximum amount of effort to produce an efficient electronic circuit, it is only natural to complement this with a high-output-current cell. 
The top six cells have been identified in the table and this mainly revolves around cost/mA output. Even if you have to pay more for a particular cell, it is best to get one with the highest output current. 
When it comes to buying solar cells or solar panels, most of your decisions will come down to the following:

  • Price
    You get what you pay for. Sometimes you get more; other times you get less. You really need to do your homework before deciding if a higher-priced cell is too expensive (given its performance), or that a cheap cell is a good buy. 
  • Availability
    You can often find really good deals on solar cells from surplus stores -- but when their supply is gone, it's gone. If you want to take advantage of such situations, you should consider keeping a reserve on hand for later experiments. If you find a high-performance cell at a good price, you'll need to be prepared to buy more. Essentially you'll have to take a stance of "buy now, design (the 'bot) later." 
  • Size
    The cell area you require will be a function of two things -- the amount of power you need (which in turn is a function of how much power your 'bot draws, and how often you want it to be active), and the performance of the cells you pick. Lower performing cells have larger cell area -- this may or may not "fit" with your desired BEAMbot design and can also cause weight problems.
  • Mass
    Heavier cells put more of a load on your BEAMbot, which usually means more of a load on your motor(s), which means more power consumption (which primarily has to be addressed by adding more cells...). Bear this in mind when you're looking at solar cell performance vs. size, particularly if you're considering buying encapsulated solar cells.
  • Cell Voltage
    Most BEAMbot designs require at least 3 volts from their solar cell(s). This means, of course, that if you buy 0.5v cells, you need to wire together at least six of them to do the job. This may not be something you want to mess with; it may not fit with the appearance you are looking for.

The table below is advertised data on solar cells sold by some of the small-cell vendors. These are  All Electronics, Electronics Goldmine, Plastecs, Radio Shack, Scientifics, and Solarbotics. Note that data is provided on 3 cell types -- regular solar cells ("C**" ID numbers), flexible cells ("F**" ID numbers), and encapsulated cells ("E**" ID numbers).

Solar cells produce 0.5v (this being a function of solar cell physics). Cells are combined in series to produce higher voltages. These are called Solar Panels. 

P1   P2      P4   P5   P6   P7   P8   P9   P10