Step 13: DTMF Decoder: Hardware

1N4001 Diode
10uF Capacitor
100uF Capacitor
0.1uF Capacitor x3
10 nF Capacitor x3
18pF Capacitor x 2
47 kOhm Resistor x2
220 kOhm Resistor x2
270 kOhm Resistor
10 kOhm Resistor x3
1 MOhm Resistor
3.579545 MHz Crystal
7805 Voltage Regulator
Atmel ATtiny2313 (Today I'd use a ATtiny 4313 or ATMega128/1280)
7-12V Wall Warts
Answering Machine or Phone that can auto-answer
RJ 11 Phone Socket
1:1 Audio Isolation transformer (still available at radioshack believe it or not!)
20 MHz Crystal
Momentary Push Button Switch
1N914 Diode
Project Box
Power connector socket to match wall wart plug.
Piece of perfboard (mine was ~17 x 28 holes)
74LS48 7 Segment Decoder (optional)
7 Segment Display (optional)
Small Ferrite beads (possible)
Screw terminals

This schematic shows how the last few circuits connect together and feed into the ATtiny2313.

The output from the 2313 to toggle the subsystems is shown at the top left of the 5th picture. These were just wirewrap sockets I could jumper to the other projects. I would update them to be wireless addressable units now using something like a zigbee , and I'd update the schematic to not use the ATtiny2313's serial pins for the DTMF input, but the circuit shown will work just fine.

One of the funny things I came across was that my CM8870 was not putting out 5V as I'd expected. It was actually floating in the range between a logic high and a logic low on the ATtiny2313, faced with it not working for my upcoming project demonstration and hacking it I chose to use hack it. I didn't have a logic level shifter or much else so I decided to drop the ATtiny2313's operating voltage which would in turn lower the threshold voltage for a logic level high.

All I ended up doing was making a voltage divider with a few spare resistors feeding the ATtiny2313's power.... horrible design I know... but it worked and got me through my review. You can probably leave this out and just connect the 2313 to 5V but be prepared to do something funny if the CM8870 output is finicky.

The last schematic should be the same as the first but without the voltage divider.