C S Technology Ltd cstech.co.uk

DTMF Encoder kit with single tone option

and TX keying.

Our DTMF encoder can encode all 16 DTMF

tone pairs directly from a keypad as well as 8

single tones.

Tones are generated for as long as a key is

pressed, and the tone audio output from the

HT9200 encoder IC is amplified, buffered and

filtered before the output level control,

improving the overall quality of the generated

signal.

A transmit keying open collector output (active

low) is provided which activates on first key

press, an inter-digit holding time of 1 or 1.5

seconds is included to keep the transmitter on

between key presses. Fit the jumper to CN4

to reduce the delay to 1 second, otherwise 1.5

seconds is used.

There is also the option of grounding the

function input (see circuit), the encoder then

encodes the 8 tones used in DTMF as single

tones when pressing buttons 1 to 8. This is

useful for checking the frequency response of

connected audio paths or

transmitters/receivers etc.

Complete kit of parts including preprogrammed

PIC micro and gold plated PCB.

The use of this keypad is optional as this kit

will work with most matrix keypads. The order

of pin out varies between keypads and this

should be checked if using a different keypad

(see circuit). The keypad can be connected via

wires if desired, often necessary with other

keypads, keep them short.

The PCB has been designed with component

pads on the back for 1nF or 10nf 0805 ceramic

capacitors for RF decoupling on all inputs and

outputs if needed, these are not supplied and

not expected to be needed except in high level

RF environments.

All parts shown in the assembled photo are

included in the kit except the keypad which is

optional.

A circuit diagram is on the last page of this

document.

Runs from 7 to 16V DC at a few mA.

Once assembled we suggest you test by using a piece of wire to link between row and column

pads of CN1 simulating the keypad action, before fitting to the back of the keypad.

Note: There may be a slight bow in the brown PCB on the back of the keypad, this is normal.

DTMF Encoder Parts List

IC1

IC2

IC3

IC4

TR1

D1, 2

XT1

R1, 2, 3, 4, 5, 6, 7, 8

R9, 10, 11, 15, 17, 18

R12

R13

R14,16

VR1

C1, 2, 3, 4, 8

C5

C6

C7

C9

C10, 11

CN1

CN2

CN3

CN4, 5

78L05

PIC16F627A (programmed)

HT9200A

LM358N

BC184L

1N4148

3.579MHz crystal

100R

10K

68K

15K

47K

10K variable

1uF observe polarity

47nF (marked 473)

100nF (marked 104)

1nF (marked 102)

470nF (marked 474)

22pf

8 pins

Not fitted

3 pins

2 pins

Also supplied:-

DTMF encoder PCB Issue A

Crystal insulator pad

Jumper

This document, product description, PCB design, circuit design and firmware

are copyright