**broken link removed**
(dont know why link doesn't work , you have to copy and paste)
so the TIL198 is just for isolation, pretend i didn't care if it was isolated... what i'm gathering is that the hdd input ("working") is connected to the ENABLE pin on the 4017... this makes sense.. so when that changes state it either starts or stops counting... but what i dont get is that +5 is connected to the ENABLE pin always... this doesn't allow for it to ever start counting... correct?
"The disable input should be low (0V) for normal operation. When high it disables counting so that clock pulses are ignored and the count is kept constant." (from http://www.kpsec.freeuk.com/components/cmos.htm#4017)
The cause of opto application: we don't know the original HDD-led circuit (LED connected to GND or to +5V) so the opto-LED connected simply serial with HDD-LED.
When the HDD-LED lite, the opto enable the counting with 555 frequency.
sebi, but it seems like the output of the optocoupler doens't do anything because +5 is connected directly to pin 13 (enable) , disabling the counting... is this correct?
david
also, the 555 isn't set up in astable mode... whats going on??
No, not directly... via 68k "pull-up"resistor. And when the opto-led lite, the output transistor can pull down the pin13 to enable the counting.
The 555 circuit probably work, if not, i'm sure You can find a better low-frequency time-base, also with 555...
When the logic input connected to positive with a resistor: this resistor called "pull-up" R. This method allow "pull down" the input with switch or transistor. The resistor need, because the logic input need logic level: 0(GND) or +(supply voltage). Floating pin make misfunction. The opto output is an NPN transistor, without LED driving not conduct, so without resistor the input floating.
Yes, this is a funny version of 555 astable. The capacitor charging from (HIGH) output via resistor, when the capacitor voltage rising to 1/3supply V the output level changed to LOW, and via resistor started the discharge.
BTW: i strongly recommend the original astable circuit of 555, don't like this cct.
NO, the resistor must be between pin13 and +, as the original draw shown. The optoLED in this case need a serial resistor about 1k. But i recommend: connect it serial with original HDD LED without resistor.
edit**
actually, i almost get it... how is it that ALL the current goes to ground when the transistor is on? Wont some of it still go to pin 13 and it kind of be inbetween high and low?