School Electronics: A great site to help you with your project and exam

Astables
Comparators
Driver Blocks
Latches

Thyristor
555 Bistable
Relay
Not Bistable
Logic Latch
Toggle Bistable
Touch Bistable

LED Blocks
Motors
PICAXE
PICAXE Inputs
PICAXE Outputs
Power Blocks
Remote Control
Sensor Blocks
Sensor Blocks 2
Sound Blocks
Switch Blocks
Timer Blocks
Touch Blocks
Alarm Example
Dice Example
Disco Example
Circuit Symbols
Handouts
Pro/Desktop
Pro/Desktop 2
Revision Notes
Links

Process: Latch Blocks Zip File ©Christopher Leigh

Thyristor Latch

This block requires an input block that goes high. This turns on the Thyristor which then stays on even when the input pulse finishes. The Thyristor can be turned off either by breaking the circuit with a push to break switch (as in the Crocodile Technology circuit) or by short-circuiting the Thyristor (as in the PCB layout). This block obviously needs an output load going to +V otherwise the Thyristor can’t latch. That's the purpose of the 1k resistor in the Croc Clips layout.

555 Bistable

This block is stable both on and off. When the 555 is triggered by a low pulse on the input, the output will go high and stay high until the reset switch is pressed. It could, of course, be reset by another input block connected instead of the reset switch.

Latching Relay

A double pole double throw relay can be turned into a latch by using one set of contacts. When the relay is turned on, one set of contacts are used to maintain the current through the relay even when the turn-on signal is removed. The push-to-break switch (actually an spdt microswitch) is used to break the circuit in order to reset the relay. The terminal block is used to connect the load and may also be used to connect an external power source (dc only). AC power (NOT MAINS) can be switched but the 0v side of the terminal block should not be connected to the pcb 0v track. The relay used is Rapid 60-0400.

Not Gate Bistable

A pair of Not gates (eg 40106) are used to form a very simple bistable similar to the Touch Bistable shown below.

Nand Gate Latch

You can use either 4011 or 4093 Nand gates to create a block which latches low when the input goes low. It is reset with the push switch or by a low input on the second gate.

Nor Gate Latch

This block works oppositely from the one above. Use two Nor gates from a 4001 to create a block which latches high when the input goes high. It is reset with the push switch or by a high input on the second gate.

Toggle Bistable

Use one D-type flip-flop from a 4013 to create a block which will toggle alternately from high to low and then low to high whenever the input is taken high. The result is a block sometimes called a T-type Flip-flop.

Touch Bistable

The Field Effect Transistor (FET) is voltage controlled unlike the current controlled Bipolar Junction Transistor. The touch bistable relies on this practically zero input current to hold the charge on the capacitor almost indefinitely. So a touch on the top sensor will turn the FET on, and a touch on the bottom sensor will turn it off. For other touch circuits see the touch sensor page.