Car engine tachometer circuit design

The automobile engine tachometer introduced in this example is made of single-junction transistors and integrated operational amplifiers as main components.

Circuit working principle: The automobile engine tachometer circuit consists of a relaxation oscillator, a differential circuit, a comparison amplifier and an ammeter drive circuit, as shown in the figure.

The relaxation oscillator circuit is composed of resistors rl-r3, capacitor cl and single-junction transistor vu.

The differential circuit consists of capacitor c3 and resistors r4 and r5.

The comparison amplifier circuit is composed of an operational amplifier integrated circuit lc, resistors r6-rlo, capacitor c2, and diodes vdl-vd3.

The ammeter drive circuit is an electronic switching circuit composed of resistors rll-rl3, capacitor c4, potentiometer rp, thermistor rt, Zener diode vs and transistor v.

The pulse frequency signal obtained from the breaker contact s of the distributor controls the working state of the relaxation oscillator and the comparison amplifier. When the breaker contact s is turned on, the relaxation oscillator does not oscillate, ic outputs low level, v is in a cut-off state, and no current flows through the ammeter pa; when the breaker contact s is turned off, the relaxation oscillator oscillates Work, the generated oscillation signal is differentially processed by c3 and r5, and then added to the positive input terminal of ic through vdl, r6 and vd3, so that lc outputs high level, v is turned on, and +l2v passes through the c and e poles of v. and rp are added to the ammeter pa (the average current flowing through pa is proportional to the pulse frequency generated by the breaker contact s); the engine speed is indicated by the ammeter pa.

Component selection

rl-r13 all use 1/4w metal film resistors. rp chooses multi-turn potentiometer or synthetic membrane potentiometer. rt uses a negative temperature coefficient thermistor. cl-c3 uses monolithic capacitors or polyester capacitors; c4 uses cbb capacitors. vdl-vd3 all use 1n4148 silicon switching diodes. vs uses 1/2w, 5·6v silicon Zener diode. vSelect s8050 silicon npn transistor. vu uses btt3 type single-junction transistors. IC uses LM339 operational amplifier integrated circuit. pa chooses lma ammeter.