eBay Chinese Electronic Clock Kit

[redwire]

Re: eBay Chinese Electronic Clock Kit

While visiting my Psychic advisor, I asked and she said the 7805 is cooked.

Before you go to run the clock from an AC adapter, pull the MCU out of it's socket and measure the 7805 puts out 5V.
If you use a 9VDC adapter and the 7805 is shorted, it would output 9V to the MCU instead of 5V and kill it. Or just power the clock from a 5V adapter.

[Rulycat]

Re: eBay Chinese Electronic Clock Kit

Could I use the NPN tester in my multimeter to test it?

[petehall347]

Re: eBay Chinese Electronic Clock Kit

test it for shorts between pin 1 and pin 3

[petehall347]

Re: eBay Chinese Electronic Clock Kit

this is the 7805 internal circuit .

[eccerr0r]

Re: eBay Chinese Electronic Clock Kit

Usually those micro tactile pushbutton switches have two pairs of leads that are connected together without pressing the button, so measure out the two pairs of pins that are connected, and have those connected pins connected to the connected traces on the bottom of the board. These switches tend to have all 4 pins shorted together when the button is depressed.

And yes I'm surprised the clock works with the dropout of the 78L05 and 4.5V input, though likely the microcontroller will work with 3V, and so can the LEDs.

I still don't get why this clock circuit needs a 12MHz clock, or is it just the silkscreen??!!!

[stj]

Re: eBay Chinese Electronic Clock Kit

it's the 8051 controller, it's usual to use it with 6, 11 or 12MHz

[eccerr0r]

Re: eBay Chinese Electronic Clock Kit

But there's nothing that dictates that the 8051 needs to be run at 12MHz, that clock would run fine at some other frequency. Software of course needs to be changed to accommodate, but so be it... save a bit of power...

[stj]

Re: eBay Chinese Electronic Clock Kit

it may have an rtc inside that needs a specific frequency for it's dividers.