Simple mistake but WHAT?

[MrDEB]

Trying to figure out why this gizmo dosn't work or at least it seems not to work.
I bread boarded most all except the resistor ladder.
looking at schematic, getting frustrated as I found several minor problems but using DMM the resistor ladder / parallel pot don't seem to give correct results.
resistance from pin 7 to pins 2/6 is supposed to change upon pressing any of the switches SW4-SW7 (the four in the resistor ladder)
gots to be something really basic but I am making it harder than it is thus preventing me from a solution.

 

[vne147]

What's the value of the R9 pot?

 

[kchriste]

The resistive ladder is a bit goofy. I would clean that up by getting rid of R6. Also remove R9 and put it between pin7 and the top of the ladder. You do realize that you must push two switches at once? ie: One capacitor switch and one resistor switch. As drawn, the circuit will oscillate at a very high frequency with no buttons pressed due to stray capacitance. The basic design needs to be rethought.

 

[vne147]

R9 was my big suspicion too. If the pot is dialed down really low or is a low value to begin with, pushing any of the push buttons in the resistor ladder will only slightly change the resistance between pins 7 and 2 & 6. And I agree, two push buttons are needed for the 555 to oscillate.

 

[dougy83]

Try just connecting up the 555 in the bog standard manner (single cap from 2,6 to gnd, resistor between 2,6 and 7) and put the resistor chain between 7 & 8. Don't connect up a parallel pot. If you want to tune it, you can connect the pot to pin 5 to change the control voltage and therefore the freq. The speaker should be AC-coupled.

 

[colin55]

Here's a simple Toy Organ circuit

**broken link removed****broken link removed**

 

[MrDEB]

Got the unit working, bad trace in the resistor ladder.
yes pressing two buttons is required and pressing more that two makes a completely different sound.
tried Mary had a little Lamb.
need to check my DMM as the current setting dosn't read. thinking fuse
contemplating removing R2 (50 ohm to speaker) as the speaker is supposed to be 8 ohms but reads 27 ohms.
Yes the ladder looks goofy but it works pretty good. Wish I could get the frequency slower as the LED dosn't flash but no big deal.
Colins circuit dosn't ave any PANASH!! lol
now I know what I need to do to get the other 11 boards to work when assembled.

 

[Boncuk]

Hi MrDEB,

connect a binary counter (CD4020) to the timer IC output and select the flash rate you want.

 

[MrDEB]

maybe do as an add on later. good idea on the cd4020
just need to configure location etc. and put in parallel with LED already mounted to boards.
might be a great add for sure.