I'm using a current sensing board from ebay to trigger an SSR, although the 'digital' output from the sensor board isn't ideal for the SSR.
I'm getting 0.2v low, and 2.2v high from the sensor board (with settings best I can figure to achieve), I want to use a transistor as a switch to send the full 5v through the SSR (3v min). The SSR draws ~5ma.
I have a BC639 (datasheet) I've found, which seems like overkill (1A max). Just from looking at transistor equations to figure out a base resistor, and whether this transistor is suitable.. I've gotten a bit confused. I'll have another look at it tomorrow, and probably get a lower power transistor from Jaycar.. Anyone with actual electronics knowledge that can shed some light on this..? - I was confused with min Hfe values - wont the base current just be lower in this case with a very low collector current?
Something isn't right.
Is that 0.2V low and 2.2V high from the sensor board, with the SSR connected to its output?
The sensor board has an MCP6041 op-amp on it, which is being used as a comparator to generate the digital output. It has a rail-to-rail output, this means with no load, the output voltage swing equals the supply voltage. In this case it has an LED and 1k resistor (these are on the board) connected to the output, which will drop the voltage a little but not much. With a 5V supply, it should give 0V low and a little below 5V high, say 4.8V, when measured with a meter.
http://www.dexsilicium.com/Microchip_MCP6041.pdfAll you need is this circuit.
http://electronics-course.com/image/bjt-switch-inductor-load.pngIn this case a solid state relay is not inductive, so the diode is not needed.
Suppose the output voltage from the sensor is 4.8V and the solid state relay draws 10mA.
Set the base current to
1/
10 of the collector current, which is 1mA.
Calculate R1.
R = (V - V
BE)/I
I = 1mA = 0.001A
V = 4.8V
V
BE = 0.6V
R = (4.8-0.6)/0.001 = 4200Ω
In reality, the resistor is not critical: 3k3, 3k9 or 4k7 will do.
To answer the original question: the BC639 is overkill but will work. The BC548 is more than adequate, but if you only have the BC639, then it's fine.