Specifications

TL/H/7451–29
TL/H/7451–30
FIGURE 13. Two Wire Remote Speed Sensor
Automotive Tachometer
Not all inputs are derived from variable reluctance magnetic
pickups; for example, in spark ignition engines the tachome-
ter is generally driven from the spark coil. An interface cir-
cuit for this situation is shown in
Figure 14. This tachometer
can be set up for any number of cylinders by linking the
appropriate timing resistor as illustrated. A 500X trim resis-
tor can be used to set up final calibration. A protection cir-
cuit composed of a 10X resistor and a zener diode is also
shown as a safety precaution against the transients which
are to be found in automobiles.
Motor Speed Controls
DC motors with or without brushes can be purchased with
ac tachometer outputs already provided by the manufactur-
er
3
. With these motors in combination with the
TL/H/7451–32
FIGURE 15. Normalized Tachometer
Output vs. Temperature
TL/H/7451–31
FIGURE 14. Gasoline Engine Tachometer
10