CHAPTER 18 ■ ADDING GEARHEAD MOTORSdiode rather than breaking down a path through another component. Schottky diodes have this desired
characteristic.
Technically, Schottky-type diodes aren’t necessary for this circuit because the transistor’s (Q7)
minimum reverse voltage is 5 V. Non-Schottky diodes have a voltage drop of around 1 V, and thus would
still provide the least-resistant path for the reverse current. Using a Schottky diode (about 0.4 voltage drop)
provides a safety margin in case some other transistor type is used that isn’t as robust as the 2907A transistor.
The little squiggly “S” atop the arrow in the diode schematic symbol indicates a Schottky diode. A plain
bar would have indicated a standard diode. A “Z” shape would have indicated a Zener diode.
Obtaining Schottky Barrier Diodes
Depending on the targeted application, sometimes a diode is called a rectifier. Either way, 1N5817, 1N5818,
and 1N5819 are popular Schottky barrier diodes. Any of them will do for flyback purposes.
The 1N5817 has the least-resistant path and can handle 20 V in reverse (see Table 18-1). The 1N5818
and 1N5819 (available from the same suppliers) have slightly more resistant paths but can handle 30 V and
40 V, respectively. Choose one of those higher-voltage diodes if you want to drive a robot with 24 V motors.
Table 18-1. Suppliers of Axial Lead Schottky Barrier Diodes
Supplier Part Number Price
Mouser 821-1N5817 $0.28
Mouser 621-1N5817 $0.44
Jameco 177949 $0.14 (per 10)
Electronix Express 111N5817 $0.25
All Electronics 1N5817 3 for $1.00
Digi-Key 1N5817-BDI $0.43
Digi-Key 1N5817DICT $0.44
Building the Motors onto the Brightness Comparator Circuit
Only four components need to be added to the brightness comparator circuit: one diode and one motor are
connected to each transistor (Q7 and Q8). See Figure 18-7 for an example.