The MT8870 is a Dual-Tone Multi-Frequency (DTMF) decoder manufactured by HALJIA with the part ID MT8870DS. It is designed to decode DTMF signals, which are commonly generated by telephone keypads, into a binary-coded decimal (BCD) output. This component is widely used in telecommunication systems, remote control applications, and embedded systems to interpret keypad inputs.
This circuit features an RTL8720DN microcontroller interfaced with a China ST7735S 160x128 TFT LCD display and four pushbuttons. The microcontroller reads the states of the pushbuttons and displays their statuses on the TFT LCD, providing a visual feedback system for button presses.
Open Project in Cirkit Designer
This circuit is designed to charge a Li-ion battery and power a DC motor and a 12V LED. The TP4056 module manages the battery charging process, while the PowerBoost 1000 and MT3608 boost converters step up the voltage to drive the motor and LED, respectively. Two rocker switches control the power flow to the LED and the charging circuit.
Open Project in Cirkit Designer
This circuit features an Arduino Mega 2560 microcontroller interfaced with a SIM800L GSM module, two fingerprint scanners, an I2C LCD display, an IR sensor, and a piezo buzzer. Power management is handled by a PowerBoost 1000 Basic Pad USB, a TP4056 charging module, and a Li-ion 18650 battery, with an option to use a Mini AC-DC 110V-230V to 5V 700mA module for direct power supply. The primary functionality appears to be a security system with GSM communication capabilities, biometric access control, and visual/audible feedback.
Open Project in Cirkit Designer
This circuit is an emergency alert system that uses a NUCLEO-F072RB microcontroller to send SMS alerts and make calls via a SIM800L GSM module, while obtaining location data from a GPS NEO 6M module. The system is powered by a Li-ion battery and includes a TP4056 module for battery charging and protection, with a rocker switch to control power to the microcontroller.
Open Project in Cirkit DesignerThe MT8870 has 18 pins, each with a specific function. The table below provides a detailed description of each pin:
| Pin Number | Pin Name | Description |
|---|---|---|
| 1 | IN+ | Non-inverting input for the analog signal (DTMF signal input). |
| 2 | IN- | Inverting input for the analog signal (used for differential input). |
| 3 | GS | Gain select pin for the input amplifier. |
| 4 | VREF | Reference voltage output (typically 2.5V). |
| 5 | INH | Inhibit pin. When high, it disables the detection of DTMF signals. |
| 6 | PWDN | Power-down mode. When high, the chip enters low-power mode. |
| 7 | OSC1 | Oscillator input. Connect to an external crystal or clock source. |
| 8 | OSC2 | Oscillator output. Connect to an external crystal or leave unconnected. |
| 9 | VSS | Ground (0V). |
| 10 | Q4 | BCD output bit 4 (most significant bit). |
| 11 | Q3 | BCD output bit 3. |
| 12 | Q2 | BCD output bit 2. |
| 13 | Q1 | BCD output bit 1 (least significant bit). |
| 14 | STD | Delayed steering output. Indicates when a valid DTMF signal is detected. |
| 15 | TOE | Three-state output enable. Controls the state of the BCD output pins. |
| 16 | VDD | Positive power supply (2.5V to 5.5V). |
| 17 | StD | Steering output. Goes high when a valid DTMF signal is detected. |
| 18 | EST | Early steering output. Indicates the presence of a DTMF signal. |
The MT8870 can be interfaced with an Arduino UNO to decode DTMF signals and display the corresponding keypress.
// MT8870 DTMF Decoder with Arduino UNO
// Reads BCD output from MT8870 and displays the decoded keypress on the Serial Monitor
const int Q1 = 2; // Connect to MT8870 Q1
const int Q2 = 3; // Connect to MT8870 Q2
const int Q3 = 4; // Connect to MT8870 Q3
const int Q4 = 5; // Connect to MT8870 Q4
const int STD = 6; // Connect to MT8870 STD (valid signal detection)
void setup() {
// Set BCD pins as inputs
pinMode(Q1, INPUT);
pinMode(Q2, INPUT);
pinMode(Q3, INPUT);
pinMode(Q4, INPUT);
pinMode(STD, INPUT);
// Initialize Serial Monitor
Serial.begin(9600);
}
void loop() {
// Check if a valid DTMF signal is detected
if (digitalRead(STD) == HIGH) {
// Read the BCD output
int bcdValue = (digitalRead(Q4) << 3) | (digitalRead(Q3) << 2) |
(digitalRead(Q2) << 1) | digitalRead(Q1);
// Map the BCD value to the corresponding keypress
char keypress = decodeKeypress(bcdValue);
// Print the keypress to the Serial Monitor
Serial.print("Keypress detected: ");
Serial.println(keypress);
// Wait for the signal to clear
while (digitalRead(STD) == HIGH);
}
}
// Function to decode BCD value to keypress
char decodeKeypress(int bcdValue) {
switch (bcdValue) {
case 0x1: return '1';
case 0x2: return '2';
case 0x3: return '3';
case 0x4: return '4';
case 0x5: return '5';
case 0x6: return '6';
case 0x7: return '7';
case 0x8: return '8';
case 0x9: return '9';
case 0xA: return '0';
case 0xB: return '*';
case 0xC: return '#';
default: return '?'; // Unknown keypress
}
}
No Output on BCD Pins:
False Detections:
STD Pin Not Activating:
Q: Can the MT8870 decode custom DTMF frequencies?
A: No, the MT8870 is designed to decode standard DTMF frequencies only.
Q: What is the purpose of the TOE pin?
A: The TOE pin enables or disables the BCD output. When low, the output pins are in a high-impedance state.
Q: Can I use the MT8870 with a 3.3V system?
A: Yes, the MT8870 operates with a supply voltage as low as 2.5V, making it compatible with 3.3V systems.