Simple squarewave LFO
I used this 555 based circuit as an LFO, to trigger the sounds in one of my bend toys. This way i can have pseudo looping. I also used it as a modulation source, by adding a photo resistor To see how, check out the description of the Ghetto Vactrol on the Carrionsound web site
The original design is a LED flasher from one of Forest Mimms books, i got it in a slightly modified form from a nice fellow on the benders list, named Justin Hassler.
Remember correct polarity for the capacitor (C1) and the LED (LED1).
If you use a socket for the 555 IC it's easier solder the two jumper wires from pin 2 to pin 6 and from pin 4 to pin 8 directly on the bottom side of the socket.
In my toy (school bus morphium thing) each sound is triggered by a switch that just connects, + on the battery to the one of the terminals on the black blob chip. So by connecting the Trigger out wire from the LFO to the terminal you will retrigger the sound each time the LED flashes. This only works if you feed the LFO with the same power supply (battery !!) as the toy it is supposed to trigger. I have tried it in toys running at 3 and 4,5 volts, without any problems, it will probably work at 6 V or 9 V too. Maybe the resistor that limits current through the LED (R3) should be increased slightly to avoid frying the LED.
Some toys use another principle for detecting which key / switch is pressed, the switches are placed in rows and columns, and some circuit does some matrix decoding. This LFO will probably not work in that type of toys, unless the way it is connected to the toy is changed. But if you follow this link to the Sailormouth web site you can find a pretty smart solution to the problem.
If you use a 100 K pot for R2 an 10 uF for C1 you get trigger intervals from about 15 seconds to insanely fast, but experiment with different pairs to suit your specific needs.
You can download a zip file with more printerfriendly version of all text and pictures on this page here.
VERO board layout :
Note : pot is shown from rear side.
Or you can use ths layout if you want to make a real PCB
This is how i connected things :
Connect + and - to the toys corresponding terminals, i put a toggle switch in one of those connections to be able to activate or deactivate the LFO
I connected the trigger out from the LFO to the center lug on a 12 pos. Rotary switch. Each of the other lugs were then connected to the 12 terminals on the black blop chip that corresponds to each sound.
If it is difficult to solder directly to the black blop, just solder to the point the original switch shorts out, but be careful, if you still wish to use the original switch.
The pot and the led is mounted in the front panel of the toy of course