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

Astables
Comparators
Driver Blocks
Latches
LED Blocks

Top LED
Bottom LED
4 LEDs
Logic Indicator
Flashing LED
Series Flashing

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

Output: LED Blocks Zip File ©Christopher Leigh

Here are four teaching and learning handouts in Word format which may help you in using these system modules. 2c Using System Modules, 5a Electronic Switches, 6b System Module Capacitors, and 7b System Module Astables. Check in the Handouts page for more useful handouts for teaching and learning.

Top Light Emitting Diode

Here an LED is connected from the input of the block to the battery voltage. A 560R current limiting resistor is used to keep the current down to a reasonable level (about 12mA with a 9 volt battery). The resistor can be adjusted to your taste. If you are using low current, high efficiency LEDs then you could use 1k or even 2k2. This block can be driven by any of the Driver Blocks or the Comparator Blocks. Here's an interesting web page about LEDs: Don's LED page

Bottom LED

This circuit connects an LED from the input to zero volts. The 560R resistor limits current to around 12mA with a 9volt battery. It can be used after some of the comparator blocks and any block that can source current, that is, any block that can connect the input to +V. All the driver blocks connect to 0v and are not suitable.

4 LEDs

As each LED requires about 2volts, if you are using a 9volt battery supply (a PP3), then you can drive 4 LEDs not just one. Again you need a current limiting resistor, here 82R. Theoretically this will give you 12mA if each LED takes 2volts. In practice some LEDs take less than 2volts so the errors in the calculation become significant. The internal resistance of a PP3 battery can also be significant. This means that the current could be much less than 12mA. So in practice you may need to use a resistor of say 47R to get a decent current.

Logic Indicator

This block provides 2 LEDs at least one of which should always be on. It is designed to act as an indicator to tell you whether the input voltage is low or high. It should work well with comparators and logic gates, but may not work with some other process blocks such as drivers. You may find that neither or both LEDs are lit. It would be better to use low current, high efficiency LEDs so that you can use larger resistors and so cut the current consumption.

Logic Indicator 2

This block uses Not gates to buffer the LEDs so that the preceding block is not loaded. It should work well with comparators and logic gates, but may not work with some other process blocks such as drivers. You may find that neither or both LEDs are lit although the use of Schmitt NOT gates should stop that happening. You should use low current, high efficiency LEDs with relatively large resistors so that you don’t load the chip too much.

Flashing LED

Here is a simple block that uses a flashing LED as the output stage. It can be driven by most process blocks. The flashing LED does not need a series resistor as it is controlled by an integrated circuit inside the casing (the black dot). The block provides a direct connection from input to output, as do the other LED blocks. This means that you can connect another output block after them. However, you may find the flashing LED acts as a process block and flashes any more LEDs connected to it. Try the block with a buzzer after it and see if it affects that.

Series Flashing LED

Although this block provides a visual indicator, it is really designed as a process block rather than an output block. Any block attached to its output will be strobed by the flashing LED if possible. So a buzzer for example should beep on and off. However, because the LED only passes a small current and takes some of the voltage, it will not work with some output blocks without a further process block such as a the Driver Block.