How do I use one output to block an input until that output turns off?
See further down for the more recently added:


A BETTER SIMPLER CHEAPER CIRCUIT :-):


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This is "out of my head" and not tested or simulated but "Should work".
The output loads are unlikely to be very high (but there is no accounting for designer insanity. This circuit should work as-is with any sensible loads and can be modified for higher loads in the very unlikely event that this is needed.  

The outputs will sink destructive amounts of current if allowed - this is unlikely to be the case but series output resistors can be added if needed. testing what output currents are is easy and should be done - place a milliamp meter in output leads. 

Summary:

Two PNP transistors are driven by the two inputs.
When a transistor is turned on by a low input signal its collector is pulled high.
This high level is coupled to the other base by a diode which prevents the other input signal reaching it's associated transistor.
An on transistor pulls up the base of an associated NPN transistor which grounds the related output lead.



Detail:

Transistors may be any "jellybean" transistors with adequate voltage and current ratings.    I prefer BC327 / BC337 if through hole, but almost anything works. 

RNa &lt;- for resistors with small letter suffix (eg R1a) - see note with same letter  

D1 D2 - if 1N4148 used then circuit should have all of R1 R2 R3 R4 in place.
If Schottky see notes a and b.
Schottky diodes can be any small Schottky diode with 30 V or higher operating voltage.  

Notes:

a. R1a and R2a omit if D1 D2 are Schottky.
OR just maybe make 100k in grubby environment if Schottky.
Values as shown for 1N414 diodes.

b. R3b R4b - short circuit if D1 D2 are Schottky.
Equip as shown for 1N4148 diodes. 

R5 R6 are nominal values. Can probably be up to 100K if note b implemented.

c. R9c R10c probably not needed except perhaps in grubby environment

d. R7 R8 10K should be OK except if output load is VERY high - which is probably not the case.



Operation:

Inputs are active low / ground to provide ground at related output.  

Idle:   

Both inputs off.  Q1 Q2 off.
D1 D2 non conducting.
Q1a Q2a off.

Input A low:

Q2 on.
D2 pulls Q1 base high either directly (R4 short) if Schottky diode used or via R4 R2 divider if 1n4148 diodes used.
Either way, Q1 base is held high and cannot be pulled low by Input B.  

Q2 on pulls Q2 base high via R7.
R7 10k is probably lower than needed in many cases but is OK.
Q2 on pulls output A low.
Max Iout is about 200 mA depending on R7 and transistor used. Hopefully output load current is FAR less than 200 mA.

Input B operation is "mirror image".  

There should be no situation where fatal mutual lockout occurs.  

E&amp;OE.
Questions welcome.







simulate this circuit &ndash; Schematic created using CircuitLab







Added:

A BETTER SIMPLER CHEAPER CIRCUIT :-):

Here is a simpler circuit which achieves the same results (in a slightly different manner).  In this case the activation of the LIGHT in one circuit disables the Button in the other circuit.
This has the advantage of locking out the 'other' pushbutton for as long as the light related to the initial button is illuminated.
This is of advantage in systems with eg PB1 &amp; PB2 and response lights L1 &amp; L2. Pressing PB1 lights and latches L1 until the system processes the button press. PB2 is locked out until L1 extinguishes.  



Two versions of the circuit are provided to cover uncertainly in how the lamp control circuits work.  

If the Light circuit goes from


Inactive - +24V to
Active (LED on) ground then  
Use the left hand circuit.


If the Light circuit goes from


Inactive - open-circuit to   
Active (LED on) ground then  
Use the right hand circuit.


In the left hand circuit (Light line at +24V when idle).  

Two identical circuits are provided - one for each Lamp/Switch circuit.

For Button ONE &amp; Light TWO

Pushbutton "SW1" ONE is able to ground its "request" line as long as Q1 is on.
Q1 is on as long as Light line TWO is high / +24V / inactive  - as L2 provides drive via R2 to turn on Q1.
If Light 2 line is low / ground / active then Q1 is off and SW1 cannot pull it's circuit low via the activated transistor.
This state continues until L2 goes to high / +24V / inactive.  

The opposite applies for Button TWO and light ONE

In the right hand circuit (Light circuit open-circuit when inactive).  

Again, two identical circuits are provided - one for each Light/Switch circuit.

For Button ONE &amp; Light TWO L2.

Operation is similar to before, with Q2 for Button ONE being disabled when Light Two is active. 

Pushbutton ONE ("SW3") is able to ground its "request" line as long as Q2 is on.
Q2 is on as long as Light line TWO is inactive (+24V or open circuit)  - as the +24V supply provides drive via R23 + R22 to turn on Q2.
If Light 2 line is low / ground / active then Q2 is off (Q2 base pulled low via D1, shunting drive via R23) and SW3 is inactivated as it is unable to pull button line two low due to the deactivated transistor.
This state continues until L2 goes to high / +24V / inactive.  

The opposite applies for Button TWO and light ONE

In this version of the circuit the fixed +24V supply provides the on drive and D1 shunts this to ground when the opposite lamp line is low.
R22 MAY not be needed depending on how close to ground the light enable circuit goes - but the R22/R21 combination allow almost any active voltage below +24V to accommodated.

Operation:

Enabled:
"Light2" input high/+24V or open-circuit:
Q2 base is pulled high via R23 + R22 and Q2 turns on.
Q2 on grounds the bottom of SW3 allowing it to ground line "Button 1 " when Sw3 is operated.

Disabled:

Line "Light 2" is grounded or taken to some voltage below +24V.
The bottom end of R23 is oulled to a diode drop (about 0.6V) above the input Voltage.
Vr23l = V_R23_Lower-end = Vin + 0.6V.
VQ2_Base_max = V23l x R21/(R22+R21) 
=  V23l/11   with resistor values shown.
With R222, R21 = 10k, 1k, input voltages below about 4V (maybe 5V) will deactivate Q2. 

If Vin_active is more than say 4V, R22 may be increased proportionately or R21 reduced.

When Q2 is active >= 0.5 mA of base drive is available to Q2 when turned on.
If an eg BC337-40 is used then in excess of 200 mA drive / ground current will be available via SW3. In many cases the "Button 1/2" circuitry will be activated by circuitry requiring perhaps 1 - 10 mA - maybe less. If a relay is activated the 200 mA+ drive current should be adequate, but if required (unlikely) a circuit with more current drive can easily be implemented. 

Q1, Q2: Almost any NPN "jellybean" transistor with a say 40V or greater collector voltage rating is likely to work. To meet the possible needs of higher current a transistor with current rating in the say 300 mA+ range and current gain in the "many hundreds" range is useful.   My favourite (there are many others) is the BC337-40 -  available from many suppliers including datasheet here from Digikey for 27 cents each in 10s. The -40 in the name indicates it is the highest current gain version - desirable. Rated at 45 V and 800 mA with a minimum current gain of 250 (400 typical) they are a very useful general purpose part.