AND gate - 4 inputs, 12V

Question

I want to make multiple AND gate input device. I have 4 locks (3 wires each; common, lock and unlock state) I want to monitor those states from locks and connect them to inputs (IC inputs) so when I have all 4 locks in the same state (all lock state or all unlock state).

This is what AND gate is for, to have one active output if all 4 conditions are same. Basically I need two IC, (AND groups) one for LOCK, and another group for UNLOCK. I want to make it as simple as possible. If anybody can suggest how to connect it, drawing, power supply is 12V and I output needs to control small 12V relay. What IC is the best for this?

Thanks in advance!

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Table 1. Truth table for relay logic. OP to edit.

A  B  C  D  | RLY1  RLY2
------------+------------
0  0  0  0  |   1     0
0  0  0  1  |   0     0
0  0  1  0  |   0     0
0  0  1  1  |   0     0
...
1  1  1  0  |   0     0
1  1  1  1  |   0     1

Answer 1

Once you get the logic worked out, if you implement it using CD4xxx chips, they should work at 12V. Then you can drive the gate of a MOSFET to drive your relay.

Answer 2

Reading between all the lines, I'm gathering you have four SPDT switches, which may either be in the LOCK or UNLOCK position. So the following circuit may be appropriate and relatively simple and very cheap. So how about using no ICs? Instead, just four BJTs and four resistors and two diodes.

I've chosen to use the Omron PCB G2RL Relay as a proxy for your \$12\:\text{V}\$ relay. It has a coil resistance of about \$360\:\Omega\$ and a coil inductance (based upon the datasheet switch timing I read) of \$1.8\:\text{H}\$. Most relays are designed to engage at 70% of their specified voltage, so the small \$V_\text{CE}\$ drops of the saturated BJTs should not be a problem. Of course, your relays can be something different. If enough different, the following circuit will need to be modified. But you'll have to pick a relay, first.

^{simulate this circuit – Schematic created using CircuitLab}

I had not tested the design through simulation before writing it out. But just to be sure it works, I did plug it into LTspice and used a 4-bit Gray code for activating the switch changes to provide coverage for all 16 permutations. (Using Gray coded switching helps avoid having to deal with "glitch" events during simulation.)

Here's the circuit I used for simulation (the two coils have a parasitic resistance equal to the relays mentioned above -- evidenced by the peak currents indicated.)

Here's the resulting information (including the state of each switch and relay):

A paltry bit of basic imagination, if I understood you well enough. I may have missed your meaning, though.

Answer 3

you can build a consensus detector like this:

^{simulate this circuit – Schematic created using CircuitLab}

Answer 4

^{simulate this circuit – Schematic created using CircuitLab}

Figure 1. Two relays are used to provide ALL UP or ALL DOWN indication. With switches in the position shown the ALL UP relay will be energised.

The diodes prevent backfeeds.

There are a couple of points to watch:

1. The diodes will cause about 2.1 V drop so your relays need to be able to energise on reduced voltage - 10 V or so.
2. When not all switches are aligned the relays will have 6 V each across them. They therefore need to be able to drop out at 6 V otherwise the last one energised will remain on.

^{simulate this circuit}

Figure 2. Modified circuit for 5 V relays. With switches in the positions show the ALL DOWN relay will be energised.

Adding the Zener diodes of Figure 2 and using popular 5 V relay coils should solve the hold-on problem.

• With the switches aligned the energised relay receives 12 V - 3 x diode - 1 x Zener = 12 - 3 x 0.7 - 5.1 = 4.8 V or so.
• With switches unaligned the relays will get \$ \frac {12 - 2 \times 5.1}{2} = 0.9 \ \text V \$ each which should be enough to guarantee drop out.

Answer 5

Now that I understand better what circuit function is desired, I have created two new ones. Figure 1shows a series connection; when all switches are in the up position relay one is on. When they are all in the down position relay two is on. The switch connection and D1 and D2 prevent either relay from acting with any other switch combination. Figure 2 shows a parallel connection; with all switches to the right relay 1 is on; with all to the left relay 2 is on.

Answer 6

When I first read the question l thought that a circuit I think of as a "4 input XOR gate" would work here. The output is high or low if all inputs are high or low. These don't exist as ICs, but can be made from resistors ,diodes and a transistor. After some thought I realized I don't know what the OP is doing, so I don't think this circuit is correct for his use.