Hot air hand dryer circuit principle and maintenance

1. Circuit Principle

　　In the circuit, V1, V2, R1, and C3 form a 40kHz oscillator, whose output drives the infrared tube D6 to emit 40kHz infrared light. When a person's hand reaches under the hand dryer, the infrared reflected by the hand is received by the phototube D5 and converted into a half-wave pulsating DC signal. The signal is coupled to the positive input of the first-stage op amp through C4 for amplification, and a small bias voltage is added to the negative end to prevent small signal interference. The amplified signal output from pin ① is shaped and smoothed by R7, D7, and C5 to become a DC signal. It is sent to the positive input of pin ⑤ of the second-stage op amp for comparison and amplification. The flip threshold of the second-stage op amp is determined by the voltage divider of R9 and R11 connected to the negative input of pin ⑥. R10 is the positive feedback resistor of the op amp, and together with C5 and C6, it forms a delay circuit to prevent power failure caused by interference when the hand being detected moves. When pin ⑦ of the op amp outputs a high level, V3 is turned on, and the control relay turns on the power of the heater and the hair dryer.
 

　　2. Common fault causes and troubleshooting

　　Fault 1: The indicator light is on after power is turned on, but no hot air blows out when you reach out your hand.

　　Analysis shows that the possibility of the fan and heater failing at the same time is very small. It is most likely that the relay is broken or not working. The non-action of J may be due to V3 not conducting; the op amp has no output; D6 and D5 fail; V1 and V2 do not oscillate or 7812 is damaged, resulting in no 12V voltage.

　　When checking, first check whether there is 12V voltage. If there is, reach out and test whether the level of pin 7 of the op amp changes. If there is a change, check V3 and the relay backward; if there is no change, check the op amp circuit, photoelectric conversion and oscillation circuit forward.

　　Fault 2: The indicator light is on after power is turned on, but the sensing sensitivity is low.

　　In addition to abnormalities in the op amp circuit, this fault is often caused by dust contamination of the red transmitting and receiving tubes. Just clean them.