Hello everyone, with the start of the new year I am sure some of you made a resolution to try a new simple modification or circuit. Even if you did not make that resolution here is a simple circuit for thought. Using a 1458 op amp you can make a triangle wave generator to drive a pair of leds that fade instead of just flash.

On a toy with a simple pitch resistor replace that resistor with a LDR (Light Dependent Resistor, call it what you want we all know what they do) and a limiting resistor in series to prevent crashing. While you are making that substitution you might want to wire in a pot in parallel with the LDR so that both the LDR and pot connect to the outer legs of a SPDT switch with center leg connecting back to pcb. The other side of the pot and LDR connections go to the limiting resistor. With that setup you can choose by the switch to use either the pot or the LDR/1458 for pitch control. Now you could leave the toy like that and use the LDR with any light source, but I like to enclose the LDR with one of the leds for the best response of the led light and darkness. The second led of the pair can be used as an indicator of the fade rate. Since the 1458 is just a light source for the LDR you can build this and use it with a toy of any voltage. They do not need to be joined to the same power source and definitely should not unless the toy also uses a 9 volt power source.

This circuit is as easy to build as a 555 to drive a led, so I built both to show the different effect of each. In my sample circuit I have a switch to select either the 1458 or 555 (50% duty cycle) to drive the pair of leds. There is also a switch to select either the pot or 1458/555. The video begins with the pot. Next it goes through the 555 at different rates, and then the 1458 also at different rates. All of those are shorter sounds, and then it goes through longer sounds with each. Lastly it goes back and forth with one longer sound. Next to the flashing/fading led is the other led with the LDR in a piece of plastic tube wrapped with electrical tape to seal out outside light. The 555 is always harsh (fast or slow rate) being full on or off, and the 1458 can give a nice warble with its fade between on and off.

[youtube]http://www.youtube.com/watch?v=VEEpZ_HVif8[/youtube]

Similar to using a 555 for noise injection you can also do the same with the 1458, again so long as the toy uses a 9 volt power source. For noise injection use a wire coming off of pin 1, the same signal that goes to the transistor.

You may be asking can this circuit run on another voltage than 9 volts? It probably can, but would require figuring out what the new values of components would need to be. It is certainly something that can be experimented with.

You may also be asking can more or fewer leds be run with this circuit? If you want to add more you should add another transistor to power them. If you use a PNP (2n3906) the two sets of leds will alternate fading. Trying to fade a single led removes the usefulness of having a rate indicator. It also means experimenting to find the right led and resistor combination to get the full on and full off fade.

Hope that gives some ideas to you. Now you should go and try them out.

by Sailormouth