AND gate problems

[mozly]

Hello,
I am trying to build this circuit straight from a book (Make: electronics by charles pratt)
here is a diagram:
**broken link removed**

Its a keypad password circuit. Basically push the buttons in the right order and the relay and the LED turns on from the output of the 555 timer.

1. My problem is that the gates dont latch
2. i get false positives from my AND gates
3. Sometimes the 555 outputs simply when i turn the circuit on?

I tried troubleshooting without success. Please help. Thankyou in advance

 

[MRCecil]

You'll need to upload a schematic in a format that is readable for anyone on this board to be able to answer your questions.

 

[mozly]

ok here you go:
**broken link removed**

or Untitled | Flickr - Photo Sharing!

 

[JimB]

Assuming that there is nothing wrong with the logic of the circuit (I have not checked it), my first reaction is that these design flaws will cause your problems:

1 There are no decoupling capacitors on the supply lines.
Put a 0.1uF capacitor between pins 7 ans 14 of the 74 series chips, and between pins 1 and 8 of the 555.
Put a 100uF capacitor between the +ve and -ve supply lines.

2 There are no back-emf suppression diodes on the relay coils.
Put a 1N4148 diode across each coil, anode of diode connects to the -ve supply line.

JimB

 

[kjennejohn]

For some reason I don't see an icon allowing me to view the pictures provided.
However, it occurs to me that the keypad switches probably need debouncing. When switches are opened and closed they tend to bounce ("chatter") as they make and break. This is usually fixed by inserting a resistor in the circuit just after the switch, and a small capacitor to ground between the switch and resistor. This RC filter "bleeds off" the chatter. Eventually the contacts close steadily, and a usable DC signal is passed to the circuits.
kenjj

 

[crutschow]

Yes, the switches need debouncing.

 

[Mikebits]

I assume the switch marked with the asterisk is your On/Off switch. It is confusing because the symbol your using is for a push button switch. Anyways, you might also want to add a power up reset to your 555, this will allow everything to be in a stable state before 555 is enabled. see diagram.

**broken link removed**