HT9170B: The Decoder Core
The DTMF Link project’s decoder relies on the Holtek HT9170B as its primary component. This IC, serving as the natural counterpart to the HT9200B encoder (detailed elsewhere in this publication), operates as a Dual Tone Multi-Frequency (DTMF) receiver. It features an integrated digital decoder and band split filter functions. Utilizing digital counting techniques via a 3.58 MHz crystal, the IC detects and decodes all 16 DTMF tone pairs into 4-bit words. Precision is maintained through highly accurate switched capacitor filters that segregate DTMF signals into low and high group signals. Additionally, a built-in dial tone rejection circuit eliminates the need for prefiltering. Remarkably, the HT9170B is pin-to-pin compatible with the renowned (and pricier) MT8870 from Mitel. Both DTMF decoder chips are readily available from Futulec (www.futurlec.com). The accompanying table outlines the correspondence between frequency pairs and the 4-bit words derived from the decoder output.
CD4099: Addressable Latch in the Circuit
In this circuit, a CD4099 functions as an 8-bit addressable latch. Data is held at the D input, and the latch’s address into which the data is to be entered is set using the A0, A1, and A2 inputs. When the Enable input goes Low, the data is transferred to the addressed output. Transitioning the Enable input from logic Low to High stores the data, leaving all unaddressed latches unaffected.
When the Enable logic is High, the device is deselected, maintaining the previous state of all latches regardless of changes in data or address inputs. To prevent entering erroneous data, the Enable should remain High (inactive) while the address lines are changing. The DTMF decoder, when receiving a valid tone pair, outputs a High on its STD pin; otherwise, it stays Low. As the latch IC2’s Enable input requires a negative-going pulse for output ‘strobing’, the logic condition is inverted using transistor T1.
Mapping Button States to Outputs
The states of individual Q0–Q7 outputs, accessible through pins on K1, represent the active/inactive state of pushbuttons S1–S9. Only one of the Q0–Q7 outputs changes its logical state. Notably, the correspondence is in reverse order; pressing S1 affects output Q7, S2 affects Q6, and so forth, with S8 controlling the Q0 output.
Output Signal Handling and Current Specifications
The output signals on K1 exhibit CMOS swing, and adherence to the CD4099’s maximum output sink/source current specification is crucial. This specification may vary among manufacturers, so consulting the datasheet is advisable if uncertain. For safe operation, low-current LEDs with commoned cathodes can be connected to K1 via 2.2 kΩ resistors. TIL199 optocouplers use the same value for their LEDs, while a MOC3020M benefits from a 470 Ω resistor. It is essential to ensure that the CD4099 outputs are not overloaded, regardless of what is connected to K1.
The encoder and decoder pair can communicate through a 2-wire line, wirelessly via an approved audio transmitter and receiver, or over AC powerlines using suitable interfaces. Both components can be assembled on a prototyping board, or you have the flexibility to design your custom PCB for this setup.