Hello,
I'm designing a circuit to ring a bell using a code set by the user. When powered the circuit steps through the code which is set via DIP switches to decide where a "ding" or a space should be within the code. Each "on" position within the code actuates a modified car relay which hits the bell. The code is 8 positions long and once finished it remains in an end state where nothing more happens so long as power is still applied to the circuit. Upon the next power on the code is reset and it starts the process again.
The circuit uses a 555 to produce the pulses (at about 1.5Hz - 4Hz) which actuate the relay arm, and a 4017 counter with DIP switches on the outputs to set the code. The output of the 555 and the DIP switches would ideally go through an "AND" gate and the result would activate a MOSFET to switch the relay on. Instead though I connected two MOSFETs in series so both have to be on at the same time to actuate the relay, the reason being that I already have a lot of them and I didn't want to buy a pack of AND gate IC's to only use one AND gate per circuit. One problem I can see with this is that the the "source" to "drain" connection between the two MOSFETs would be left floating, so I pulled it ground via a 39k resistor.
Other than that though is there anything wrong with doing it this way? I've tested the circuit on breadboard and it works well, but I didn't know if there might be any potential problems doing it like this. I have plenty of 2N3904, 2n3906 and BC337-25 transistors so if there is a better way to perform the AND logic with discrete parts please let me know.
The circuit runs off of a 12V 1.25A DC Power adapter, and the car relay is rated at 12V 200mA.
I've attached the full schematic below:
Thanks in advance for your help.