The reminder circuit is as shown. It uses a 555 time base circuit to complete both delay and oscillation functions. The circuit is simple and practical. The delay function can be realized by using the reset terminal 4 of the 555 to connect the capacitor C6 and the relay contact K1-1 to the ground. Desktop Phone Holder,Desktop Mobile Phone Holder,Adjustable Desktop Phone Holder,Universal Desktop Cell Phone Holder Ningbo Luke Automotive Supplies Ltd. , https://www.nbluke.com
555 and R2, R3, C4, etc. constitute a 15 second delay audio oscillator. The SA in the figure is the gate (illumination) switch of the original refrigerator. When the refrigerator door is opened, the SA is automatically closed and the control circuit is energized. The AC power is stepped down by the C1 step-down capacitor and is rectified and stabilized by the full bridge QU as the supply voltage of IC2. When the refrigerator door is just opened, the relay K is pulled in, and its contact Kl-1 is disconnected. However, since the voltage of C4 cannot be abrupt, the 4 feet of 555 are low, which makes 555 forced reset, that is, 555 does not work. Thereafter, with the charging of C4 (via 555 internal circuit), when the charging voltage on C4 reaches about 1.4V, 555 is turned on by resetting and set to start, and the audio pulse outputted by 3 feet stimulates the speaker to emit an audible alarm. , remind users to close the door in time. The audio oscillation frequency f is f=1.44 ∕(R2+2R3)C4.