4017B Decade Counter

[Paul_77]

I am trying to make a circuit to flash LEDs on and off using a 555 timer and a CMOS 4017B decade counter chip.  Whenever I try to connect one LED to two of the outputs in parallel, all the outputs start going high at the same time - the chip has stopped counting.  I have tried separating the two outputs with diodes and pulling all unused outputs down to 0V with diodes but nothing fixes the problem.  Can anyone help?

Thanks
Paul

[audioguru]

Hi Paul,
A 4017 has outputs that can go low or high. If you short together outputs then a high current flows inside. If the supply voltage is about 10V or less, no damage was caused.
Use a diode in series with each output to isolate them from each other then connect the free end of the diodes together and to your LED. The diodes perform like a "Mickey Mouse" OR gate.

1) Each time an LED is lighted, the supply current is increased which might cause the supply voltage to sag.
2) Also, an ordinary 555 causes high current spikes (400mA) from the supply each time it switches, because it is designed with outputs capable of supplying 200mA to a load.
Both problems might cause the 4017 to lose its count. They can be solved with a "stiffer" supply and/or a supply bypass capacitor to hold up the supply voltage when current is trying to make it sag. ;D