You'll need good eyesight to solder that one !

Edit: Depending on the start-up time of the power supply and computer you may need to add a power-on reset to the circuit I posted to ensure a consistent Q output. Something like this:

 

You'll need good eyesight to solder that one !

Hi,

eyesight sucks, but the magnifying glass works. but i can handle the 4 sided cyclone II 208 pin pqfp so this one will be a breeze

k, seen your edit, i'll post my results

 

Oops. My bad. The circuit I posted would be ok if the LDR started dark and subsequently got bright; but not if vice versa. So, a similar problem to your circuit. Back to the drawing board methinks, to find a better latching arrangement.

 

Right, this should be better (unless I've overlooked something) ....

 

Right, this should be better (unless I've overlooked something) ....
View attachment 75128

Hi Alec,

Thanks for the update, i think i may have a couple of these kicking about

how does this substitute look like ?

http://www.ti.com/lit/ds/symlink/sn74lvc1g373.pdf

 

Looks fine. I didn't know there was a single latch version available.
By way of explanation, the latch is enabled (thus Q0 takes up the state of the LDR) only during the first ~200mS from power switch-on. I'm guessing that will be long enough for your purpose, but R2 can be increased if necessary to extend the time.

Edit: R1 could be a trimmer (say 100k) if you need to adjust the light sensitivity.

 

Looks fine. I didn't know there was a single latch version available.
By way of explanation, the latch is enabled (thus Q0 takes up the state of the LDR) only during the first ~200mS from power switch-on. I'm guessing that will be long enough for your purpose, but R2 can be increased if necessary to extend the time.

Edit: R1 could be a trimmer (say 100k) if you need to adjust the light sensitivity.

If i have a latch kicking about i will try it today, otherwise it won't be until friday. In theory, if this works i'm thinking i do not need the And gate portion as this chip would satisfy the ROM select signal either by holding it high or low. It would be great if it makes my circuit so much simpler

tantalum or aluminium capacitor for the 10uf or does it not matter ?

 

You won't need the And gate. A tant cap would be smaller, but otherwise either type will do.

 

You won't need the And gate. A tant cap would be smaller, but otherwise either type will do.

I just knocked up a temporary circuit using a single latch and Success!! circuit works as is without even using a trimmer, for the final circuit board i will set it up for a trimmer and a pad for a resistor in parallel so there would be a choice to use either a fixed or a trimmer. It will need tweaking to find the best levels but it appears to have a good sensitivity as is. Once the power is turned on the latch will lock its output and the output doesn't wobble (nice electrical term eh . The final board will be about 0.5" square and have a 3 wire connection to the ROM so it can be placed anywhere in the case that can be a good source of seeing the ambient light (Vents, gaps etc), and even drill a hole if people want

Once i get the final PCB done i will post a little video. Thanks for your help Alec, so simple in the end

Thanks again!

 

Good to know you've got it working.