With help of this site, I have redeveloped my voltage alarm circuit (see my previous question for the history). I started off with an 741 Opamp and it did not work as I wanted it to work (Thanks for your input here). So I did some research and found a LT1017 comparator.
Based on the data sheet, I assumed that I can drive a LED with the output (it says in the datasheet that it can source 70 mA). But it does not work. The output voltage collapses under load.
Anyway, I switched it around and used a PNP transistor (2N2907) to drive a relay, LED and buzzer (See schematics).
So I still have issues and I don't understand why. The 2N2907 has a hfe of around 250 (measured with my multimeter). The buzzer, relay and LED use about 70 mA. Let's keep it simple and use a hfe of 100. This means I need 0.7mA as ib and with 14V Vcc we are at 14-0.7 at 13.3V/0.7mA, that would be 20k.
My problem: With this calculated Rb nothing happens. Even with Rb = 1k only the relay and LED works, the buzzer does not produce sound.
If I play with Rb and measure ib and ic, I get ib of around 10mA and a ic of 50mA. So the transistor has a hfe of 5 according to this.
I know that I must be making a massive error, but where is it? I learned that stuff 30 years ago and I am going back into it now a little. Of course I forgot many many things from back then, but what am I doing wrong?
Thanks in advance for your input.