I've been looking for a way to control a latching relay with a microcontroller that doesn't quite have enough current on the output pins to operate it directly. I found this intriguing circuit using a 220uF capacitor and an NPN transistor, but haven't had any luck getting it to work as described.
The circuit I'm referring to is the first one on this page, labeled "Pulse Latching Relay On/Off":
Here it is below:
From the circuit description:
The circuit above produces a strong pulse to latch the relay ON and the input voltage must remain HIGH. The 220u gradually charges and the current falls to a very low level. When the input voltage is removed, the circuit produces a pulse in the opposite direction to unlatch the relay.
Problem is, the circuit doesn't work. They don't clearly label the inputs, but I gather that top is (+) and bottom is (-), and the 10K operates as a pullup, holding the transistor open to discharge the capacitor. When the lower input is connected to ground, the transistor switches off, and here's where I get confused. Is there supposed to be electron flow from base to emitter to charge the capacitor via the coil? Seems that would reverse-biase the BE junction and current wouldn't flow.
My attempts to breadboard this with a T2n3904 and a 470uF electrolytic have failed.
Is this circuit bogus or am I just stupid?