TWO-TONE

DOOR BELL

INSTRUCTION SHEET

 

Price - Stock No 240

 

PCB Layout

 

  1. Inspect the tracks for fine breaks and test the continuity of each track with  an  electronic  circuit  tester  or  ohm-meter.  
  2. Bend  the  legs of the resistors to match their respective holes and push them through from the plain side of the P.C.Board.  Identify the 180K resistor, all the other resistors are 33 K.  Solder in place - refer Soldering Technique   
  3. The three 1N4002 diodes are can be bent to shape and  mounted  with their band to the K end of the space. - see the placement sheet.
  4. The 8 pin  IC  socket can be pushed carefully into place. Solder in place.
  5. There are three electrolytic capacitors.   Their  values are  marked on the body, and there is also an arrow pointing down one  leg.  This leg is the negative leg.  Polarity must be observed.  The .022nF capacitor can go in any way round.
  6. Two pins can be pushed into the holes where the speaker connects and two more where  the push-button switch goes.  These can be soldered off.  The speaker is connected by means of the Fig.8 wire.  Cut the fig 8 wire in two suitable lengths, bare and tin the ends of the wire - see refer Tinning wire.   Attach the speaker and switch taking care to ensure the speaker's polarity is observed.
  7. The 2N7000 is now placed in accordance with the PCB Layout diagram, solder in place.
  8. Find the locating DOT on the 555 IC and arrange it as the  drawing shows.   To  fit the 555 it is best to use an IC inserting tool but if one is not available it will probably be necessary to bend the legs inwards  a little by placing one set of legs on a flat surface and push down lightly.  This will maintain their alignment.  Repeat for the other side of the chip so they match the socket.  Push the IC into place.
  9. The  battery  holder  is mounted from the TRACK side.  This will give the correct polarity.   Push the battery  holder  into  place  and  solder. Check your two joints with a continuity tester. When  the battery (9Volt) is placed in the holder you should hear a click from the speaker.  Push the button and release it and you  have  two distinct tones.

Trouble shooting

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.

Technical notes

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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