Print PDF
Favorite
Email
Step 3Circuits
To control the door strike, I used the relay I found in the coffee percolator. This relay needed more than 5V TTL to drive it, so the sn754410 was used to translate TTL to 19V, which drove the relay. The sn754410 is really a quad half-H driver, so I was wasting 3/4 of the chip, but I didn't have any power transistors, so that's what I used.
The sn754410 chip has two VCC pins, one for 5V, the other for what ever voltage you want to come out, which was 19V for me. It's a really cool chip. You can use this to drive motors and relays directly, because it can switch 1A per quarter chip and has built-in protection diodes. Take a look at the datasheet. In my circuit, I connected the sn754410 directly to my Teensy's output pin.
Buttons are connected as active-low, which is very common for microcontrollers. They are directly connected to the Teensy, which means I have to do debouncing in software.
The status light is connected to the Teensy through a 1K ohm resistor; nothing special.
The circuit worked without capacitors, but I put them in anyway just in case. There are protection caps on both the 19V and 5V power rails to ground.
While programming the Teensy, 5V came from the USB, but when it's running on its own, the power comes from the laptop power brick. When I connected the 7805 regulator directly to 19V, it got REALLY hot, so I put in a network of resistors to limit the input voltage and current to the regulator. This was a kludge, but now everything is at a manageable temperature.
The sn754410 chip has two VCC pins, one for 5V, the other for what ever voltage you want to come out, which was 19V for me. It's a really cool chip. You can use this to drive motors and relays directly, because it can switch 1A per quarter chip and has built-in protection diodes. Take a look at the datasheet. In my circuit, I connected the sn754410 directly to my Teensy's output pin.
Buttons are connected as active-low, which is very common for microcontrollers. They are directly connected to the Teensy, which means I have to do debouncing in software.
The status light is connected to the Teensy through a 1K ohm resistor; nothing special.
The circuit worked without capacitors, but I put them in anyway just in case. There are protection caps on both the 19V and 5V power rails to ground.
While programming the Teensy, 5V came from the USB, but when it's running on its own, the power comes from the laptop power brick. When I connected the 7805 regulator directly to 19V, it got REALLY hot, so I put in a network of resistors to limit the input voltage and current to the regulator. This was a kludge, but now everything is at a manageable temperature.
| « Previous Step | Download PDFView All Steps | Next Step » |
 |
Add Comment
|
