- Joined Dec 8, 2021
It continuously beeping as soon as LDR turned on. which is weird it was supposed to wait atleast 20secIs the voltage at the R1/LDR junction being pulled down below ~4V when the LDR is illuminated?
Is the LDR illuminated continuously for at least 20 sec, to allow C1 to discharge below the Trig threshold?
By my reckoning, the combination of R2 and R3 should be about 370k for a 20 sec delay.
That is how they work!! I still think that the first timer circuit is not right. The first timer need to trigger on a falling edge.@enciklinux
What you want the circuit to do is sound the buzzer if the light stays on for more than 20 seconds continuously, is that correct? In other words, someone accessing the fridge (keeping the fridge's door open) for less than 20 seconds won't trigger the alarm, but if the door is not fully closed and thus the refrigerator's light stays on for more than 20 seconds, then the buzzer should sound.
D1 is needed so that C1 discharges at a known rate via R2 and R3 (which combined are actually too high in value for a 20 sec delay), instead of discharging through the unknown/variable resistance of the LDR.Replace diode D1 with a piece of wire
I initially thought that, but it simulates ok (though the timing needs adjusting).The circuit for the first timer does not look right
The description sounds reasonable, I guess, but the TS complains that it does not work that way. It might also have something to do with not having that capacitor on pin 5 that all of the application literature always shows.The U1 part of the circuit is unconventional but actually does the job. Here's how :-
1) Before power-up, C1 is in the discharged state and Q is low.
2) At power-up, with LDR non-illuminated, C1 charges up rapidly via R1/D1 and crosses the Trig threshold. Q is now high, so there is a second charge path for C1 via R2 and R3.
3) Q remains high (and hence the buzzer activates) for the brief time it takes for C1 to reach the Thr threshold.
4) Q now goes low (turning the buzzer off), but C1 remains charged to near the supply voltage, because R1 is much less than R2+R3.
5) Only when the LDR is illuminated can the charge path via D1 be blocked. This then allows C1 to discharge via R2/R3.
6) Provided the illumination is maintained for at least the required period the discharge takes the Trig pin below its threshold, allowing Q to go high again to re-activate the buzzer.
LTspice says it does work that way. If the TS is only allowing the light to be on for 20 secs then it won't work with the timing component values shown in the schematic (which give a delay of ~33 sec).The description sounds reasonable, I guess, but the TS complains that it does not work that way.
That indicates D1 is shorted or C1 is open or not connected.It continuously beeping as soon as LDR turned on. which is weird it was supposed to wait at least 20sec
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