How can I use a transistor to "press" a button?

Question

I have a heater remote that I want to press the buttons of with my Arduino. I am powering both the remote and Arduino from an external 5V source so both grounds are connected.

Currently I have tried using an NPN transistor(TIP122) and it does work but is not very reliable. It probably works 80% of the time but isn't good enough for my purpose. Sometimes nothing happens and sometimes the button is pressed twice for no reason.

These are my connections.

I have the emitter connected to ground, collector connected to the button and the base connected to the Arduino with a 1k resistor. Are my connections OK?

I've also tried using a PNP transistor but pulling LOW instead of HIGH and LOW instead of HIGH.

The code is very simple, in the setup I have:

pinMode(4, OUTPUT);
digitalWrite(4, HIGH);
delay(20);
digitalWrite(4, LOW);

Therefore when I restart the Arduino it should press the button, right?

I also have IRLZ44N and IRF540N, would either of these work better?

Answer 1

A glance at the TIP22 datasheet shows that this is a Darlington pair transistor, which means that it has very high DC gain (good), but at the cost of a very high Vbe_ON (bad). The datasheet shows that the Vbe_ON is 2.5V, which means that the voltage across the 1K is only 2.5V max, which limits the base current to around 2mA. This is probably not enough for good saturation. In addition, the Vce_SAT for this device is about 2.0V, which may or may not be enough to look like a good GND to the button circuit.

I think a MOSFET like the IRF540N, connected as shown here (substitute button circuit for motor) might be a better choice YMMV

Answer 2

You really only need one wire and no resistors or mosfet.

I see many answers including expensive mosfets and relays to answer this question. There is a much simpler solution requiring no glue logic or interfacing circuitry. I recently had to install an off-the-shelf ultrasonic humidifier in an incubator/brooder. It is the cheap little home-use aromatherapy type diffuser. It had to be controlled by a relay to turn on and off. The catch is that the control circuit and firmware used in nearly all of these is the same, it can be turned on by a relay in the PID incubator controller but requires a single button press once power is supplied to start the ultrasonic humidifier and fan. three button presses turns the humidifier off and returns it to standby and turns off automatically when it runs out of water. using the arduino to control only the button to bring it out of standby would not lead to reliable operation. I searched for an answer but found none that I could make given I had few parts and was working on a remote farm. This is my solution.

The science behind this and how it works is that the microcontroller does not sense the press of a button, it detects a state change. You may familiarize yourself with this in the arduino tutorials for reading a button press.

Tie the arduino ground to ground in the circuit you want to control. find a 5v rail and power the arduino. no extra power required. There is usually a test pad that will be ground and 5v close to the button you want to control. if not, find the regulator for the microcontroller that you are trying to trigger by tracing the circuit and attach the power rails. I used Digital Pin 2 for the trigger signal. connect d2 to a point near the switch. Ensure there is a resistor of about 600-1000ohm in the circuit between the point and the microcontroller. This should be present on the circuit you are triggering already simply because this is how microcontrollers detect state changes without sinking or sourcing infinite current.

This code will trigger once and then go to sleep. The long delays are to allow for expected debounce code in the microcontroller and allow for boot time. put simply, it switches PD2 between high and low states in high impedance and low impedance states and basically all a switch does is switch between high impedance and low impedance states. This code should trigger pretty much any microcontroller regardless of the specific state required once and then go to sleep.

#include <avr/sleep.h>
void setup() {
  pinMode(13, OUTPUT);
}

void loop() {
  pinMode(PD2, INPUT_PULLUP);
  digitalWrite(PD2, HIGH);
  delay(2000);
  pinMode(PD2, OUTPUT);
  digitalWrite(PD2, HIGH);   
  digitalWrite(13, HIGH);
  delay(300);
  digitalWrite(13, LOW);
  pinMode(PD2, INPUT_PULLUP);
  digitalWrite(PD2, HIGH);   
  delay(300);      
  pinMode(PD2, OUTPUT);
  digitalWrite(PD2, LOW); 
  digitalWrite(13, HIGH);
  delay(300);
  digitalWrite(13, LOW);
  pinMode(PD2, INPUT_PULLUP);
  digitalWrite(PD2, LOW);    
  delay(500);
  sleep_enable();
  set_sleep_mode(SLEEP_MODE_PWR_DOWN);
  sleep_cpu();
}