Price - Stock No 240
Troubleshooting if necessary will involve careful checking of locations and polarity of components, mainly the electrolytic capacitors, the diodes D1,D2,D3 and that the locating DOT on the 555 IC is as the drawing shows. Re-solder all joints and check to make sure you have not bridged across between any two adjacent component legs. The end of a broken hacksaw blade sharpened on an emery wheel is a good tool for cleaning between soldered joints.
The circuit contains an auto-shutoff feature. You can hear the circuit power down if you listen carefully after the ding-dong, a faint tick can be heard about four seconds after releasing the button.
We will look at this first. The 10uf capacitor, the 180K resistor, # 1 - 1N4004 and the 2N7000. When the switch is depressed, current flows via the 1N4004 filling the 10uF capacitor and turning the 2N7000 on (at the Gate). When the button is released this system is isolated by the 1N4004 and the 10uF capacitor begins to drain via the 180K resistor. At about 4 seconds, the capacitor/resistor system has emptied suffeciently to switch off the 2N7000. This measure effectivly reduces the current draw to zero, ensuring a long battery life.
Let's now look at the ding part of the ding-dong. When the button is depressed current flows through # 2 - 1N4004 to pin 4 of the 555, enabling the IC. This also fills the 4.7uF capacitor. The ding sound is now generated by the 555, appearing at pin 3 and commuted to the speaker through the 100uF capacitor. The frequency of the 555 is determined by the 22nF capacitor and the network of 33K resistors leading to +9V.
So while the button is depressed (the doorbell is ding-ing)
C = 22nF
RA is 33 K *
RB is 33 K
* While the button is depressed, the top 33K resistor is short circuited through # 3 - 1N4004. So it does not come into this equation. See Resistors .
(RA + 2 RB) = 33 000 + 66 000 = 99 000
(RA + 2 RB) C = 99 000 x 0.000 000 022 = 0.002178
f = 1.44 / 0.002178 = 661 Hz
See Scientific notation.
When the button is released, two things occur, the top 33K resistor now comes into play and the 4.7uF capacitor now activates the 555. The 4.7 Uf capacitor now drains through the 33K resistor in parallel with it and determines the duration of the 'dong'.
Now using the above equation
C = 22nF
RA is 33 K + 33 K = 66 K
RB is 33 K
(RA + 2 RB) = 66 000 + 66 000 = 132 000
(RA + 2 RB) C = 132 000 x 0.000 000 022 = 0.002904
f = 1.44 / 0.002904 = 496 Hz
So the 'ding' is at 661 Hz and the 'dong' is at 496 Hz.
As discussed earlier, the 2N7000 switches off after about 4 seconds to conserve battery.
View the Datasheet for the 555 .
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