Hello World (capacitor timer?)

Nov 14, 2012



#1020 Nov 14, 2012

Last evening I did my first transistor circuit..........   Amazing considering the many projects involving electrical stuff I've builtsince my pre-teen years (60's).   

I received my packet of 10 IRF640 n channel mosfets in the mailyesterday.   (high voltage moderate current capacity), and proceededto cobble together the basic "hello world" circuit where you justflip a switch and the mosfet lights a small bulb.    I left out thecapacitor between gate and source just to see how long it would taketo shut off.  It seemed like it probably would stay lit for manyhours if not days....    A 470 ohm resistor instantly killedit.      This am I pulled the 470 and installed a 10M, thinking Imight be able to see the delay.....NO WAY... instant.   I theninstalled a 10 uf capacitor I had laying about in parallel to the470 resistor, and achieved almost exactly a one minute oninterval.   I'm assuming that if I install a capacitor of doublevalue, the delay will be twice as long, and likewise if I increaseor decrease resistor values.     Also removed the switch andinstalled a rheostat to dim the light......just for play.

    This shows me that I can do my "latching relay logic" by simplyusing mosfets and capacitors.  One mosfet can latch using a cap,another transistor on the appropriate signal can short the cap toground unlatching it, and at the same time latch itself.  

    I had no real idea what I was missing by not learning how to usethese kinds of components.   I have P channel mosfets coming as wellas a prototype board with power supply, and a half dozen LM555s formy project.   I hope to play with more circuits.  I want to tryusing an N channel and a P channel, running a load on the N channel,and pulling a circuit between the drain of the N channel and thegate of the P channel, putting a dummy load on the P channel (highvalue resistor to ground) on the drain side of the P channel,putting a circuit back to the gate of the N channel between thedrain and the load, with a normally closed switch between.    Thisseems to me like it should create the latching relay circuit I waslooking for.

    Lacking a prototype board, I just cobbled this stuff togetherusing some scrap parts, and crudely soldering everything using thelegs of the mosfet as solder terminals, and a needle nose vice gripas a heat sink.   Later in the week, I expect to have on of thosecool solderless breadboards and a plug in USB powered power supplythat attaches directly to the board so I can make cool looking testcircuits.  My injector tester circuit will be prototyped on thisboard initially, then transferred to a solder type breadboard.  

    Here are some of my crude "hello world" project and a schematicof my latching system with two mosfets (p channel and n channel), atthe group web site:    tinyurl.com/a33jxbv



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