I'm very new to circuit design and have been doing lots of reading and research in order to understand everything going on in this schematic. I think I understand all of it except for the purpose of the 1k ohms resistor going from the control signal to the emitter.
Can someone explain what that does?
This circuit is part of a bigger project outlined here: http://victorbush.com/2015/01/arduino-rfid-door-strike/
The 1k resistor is a pull-down resistor. Not the weakest one of that, but that's okay. It is meant to pull the base of the transistor to a known state (ground) when the control signal is missing/open/high input. To prevent a floating signal or even leakage current from turning the transistor on, to make sure the strike doesn't open or engage randomly. It does form a small voltage divider, but not enough to really affect the transistor performance. Due to its value, it will waste 0.5mA to ground when the control signal is high, so it's value may need to be changed for battery applications. A 10k to 47k may be a better value.
I'm not saying this is what was in the designer's head, but it's possible that with the Arduino microcontroller not powered a powerful RF signal could be rectified by the transistor base and cause the door to unlock or heat up the transistor and solenoid. It also reduces the leakage current if the transistor was hot and the Arduino was not powered. It doesn't do much of anything useful if the Arduino is powered and the output is configured to pull down.
The 1K takes 0.7mA of the available base current only when 'on' (a few percent probably, and less than 0.3% of the total current including the solenoid) so it's reasonable enough. If replaced with a larger value resistor, a capacitor from base to emitter would eliminate the RF concern. To pick a reasonable value say that Xc = (say) 1000 ohms at (say) 100kHz so a few nF would do the trick.
However there appears to be a flaw in the ointment (sic) here- the solenoid inductance is a powerful storage reservoir of magnetic field energy that will cause the voltage to rise to hundreds of volts and thus break down the TIP3055 when the power is shut off to the strike. To avoid this, a reverse-biased diode should be installed across the solenoid. It's possible there is one already inside, but I see no mention of it on the datasheet or install sheet.
A 1N4004 or similar will do the trick. The TIP3055 is a fairly robust device and it will probably survive this for a while, but it would be a major pain to have it fail (probably on)- it appears to be rated at 30 seconds maximum 'on' time- and burn out the expensive strike mechanism, whilst leaving the door open for rascals to ransack the place.
This resistor performs two functions.
1) It is a pull-down resistor.
2) It is part of a voltage divider. This allows the threshold voltage to be set to the desired value.
For example, it is possible to pick resistor values such that the transistor doesn't turn on until say, 120 Volts is present at the circuit input. Almost any voltage greater than Vbe can be selected.
This allows you to improve resistance to false or low-level signals.
Note that the threshold voltage is sensitive to both temperature changes as well as variation between devices. Nonetheless, it is an important design element.
