Schematic Power Amplifier and Layout

Circuit control temperature fan ceiling

P1_____________22K

R1_____________15K

R2____________100K

R3,R6__________10K

R4,R5__________22K

R7____________100R

R8____________470R

R9_____________33K

C1_____________10nF

D1________BZX79C18

D2_________TIC106D

D3-D6_______1N4007

Q1,Q2________BC327

Q3___________BC337

SK1__________Female Mains socket

PL1__________Male Mains plug & cable

R3-R4 and P1-R1 are wired as a Wheatstone bridge in which R3-R4 generate a fixed two-thirds-supply “reference” voltage, P1-R1 generate a temperature-sensitive “variable” voltage, and Q1 is used as a bridge balance detector.
P1 is adjusted so that the “reference” and “variable” voltages are equal at a temperature just below the required trigger value, and under this condition Q1 Base and Emitter are at equal voltages and Q1 is cut off. When the R1 temperature goes above this “balance” value the P1-R1 voltage falls below the “reference” value, so Q1 becomes forward biased, pulse-charging C1.
This occurs because the whole circuit is supplied by a 100Hz half-wave voltage obtained from mains supply by means of D3-D6 diode bridge without a smoothing capacitor and fixed to 18V by R9 and Zener diode D1. Therefore the 18V supply of the circuit is not true DC but has a rather trapezoidal shape. C1 provides a variable phase-delay pulse-train related to temperature and synchronous with the mains supply “zero voltage” point of each half cycle, thus producing minimal switching RFI from the SCR. Q2 and Q3 form a trigger device, generating a short pulse suitable to drive the SCR.