I believe this is the final variation on the door minder circuit. Because there wasn’t sufficient current from the encoder chip, we must drive the LED directly from the batteries, using a BJT, in this case, the 2N3904. The new LED, SFH4511, has a smaller half angle and higher radiant intensity than the TSAL6200, and better serves us at the distance we need to transmit the signal. Because we only need to see if the signal is interrupted, we will only transmit one bit, D0.
I am currently testing the circuit with the LED on top of the receiver, inside a black heat shrink tube in an orientation something like this:
The heat shrink tubing is needed because the receiver can still pick up infrared waves from the LED just being turned on. Another possible problem is that at distances of about 2 feet from the circuit, the receiver can pick up reflections off of skin and clothing.
An equivalent circuit to LED driver (driven from output of encoder chip) when on:
We use 0 for the resistor between ground and the emitter node. The power dissipation of the LED is 165mW, and the current through the LED is given by:
i_led=1.5v/R_led
Therefore, because P=vi, and we know the forward voltage drop for the LED is about 1.3v, R_led must be around 12 Ohms:
P=vi => 165mW=(1.3v)*(1.5/R_led) => R_led=1.95/0.165 Ohms = 11.8181 Ohms.
The base current can be calculated as (3v-1.5v+0.7v)/100Ohms=8mA. The emitter current is the current through R_led, 1.5/12=125mA. Adding these, the circuit uses 133mA when on.
The signal takes 27-28ms to be sent (see previous posts), so we should leave the encoder toggled for around 30ms, just to be safe. If we repeat this every 100ms, the encoder will be on 30% of the day. The batteries we use claim to provide 1500mA hours. Because 133mA is being used 30% of the time , we can use 30% of 133mA to calculate the circuit’s life. This is 1500mA*h/39.9mA=37.594h, about one and a half days.
We could turn on the LED every 200ms, and double the life to a little over three days (75.188h), but we will need to make sure the decoder receives every signal.
The new LEDs will arrive soon, it may be possible to get reliable results with less power, but previous experiments do not suggest this.