TimerCircuits555 Help.
TimerCircuits555 is a simple electronics application that simulates bistable, monostable and astable circuits implemented with a 555 timer IC (e.g. NE555). The following help text only concerns the usage of the application. For details concerning the 555 timer and the circuits simulated by the program, please, search the Internet or have a look at an electronics book. The 555 and 556 Timer Circuits page at the website of The Electronics Club, for example, contains all information, that you need, to understand how the IC and the circuits work and how the time period (monostable) and mark and space time (astable) are calculated.
Exit.
Exit the TimerCircuits555 application.
With the items in this menu menu, you can make some specific settings for the different circuits. The Output LEDs settings apply to all three circuits. The warning messages option applies to the calculation of the resistance(s) of monostable and astable circuits.
Bistable circuits.
Selection, how the bistable circuit should be implemented, concerning the reset of the flip-flop: by using the Reset pin, or by using the Threshold pin.
Monostable circuits.
Two selections that you can make here:
- Adding or not a second switch to reset the timer at any moment.
- Choosing what value you want to calculate: the time period you get for a given resistance or the resistance you'll need for a given time period (the capacitance having always to be given by the user).
Astable circuits.
Two selections that you can make here:
- Choosing if you want or not a diode mounted in parallel with R2 (making possible duty cycles less than 50%).
- Choosing what value you want to calculate: the mark and space time for given resistances or the two resistances you'll need for given mark and space time (the capacitance having always to be given by the user).
Output LEDs.
The output of a standard 555 timer can sink and source up to 200mA. This is sufficient to supply many output transducers directly, in particular LEDs (with a resistor in series). The options of this menu item allow you to choose if you want to connect the LED to +Vs (LED on if output is low) or to 0V (LED on if output is high), or using 2 LEDs (getting a blinker circuit in the case of the astable). Note that in the circuit display, a colored circle is used to represent the LED plus its resistor.
Don't display warnings.
The application considers that all resistances must have standard resistor values. If the time period is calculated and you enter a non-standard value, you get an error message. If the resistance(s) is (are) calculated, the time period resp. the mark and the space time are adjusted by the application in order to get standard resistor values. To suppress the warning message, displayed in this case, check this option.
Help.
Displays usage help for the TimerCircuits555 application (this text) in your webbrowser.
About.
Displays version, author and date-written of the TimerCircuits555 application.
Bistable circuits.
Bistable circuits (flip-flops) are memory circuits with two stable states. Pushing the SET switch, the output of the flip-flop becomes high and LED2 (LED connected between pin 3 of the 555 and 0V) goes on; the output remains in this state, even if you release the switch. With the RESET switch, the output of the flip-flop becomes low and LED2 goes out; the circuit remains in this state after the switch has been released.
Monostable circuits.
Monostable circuits have only one stable state (default state, normally the one where the circuit output is low). The other state is unstable, i.e. after a given time (called the time period) it automatically switches back to the stable state. The time period depends on the time it takes to load the capacitor at a given charge, thus depends on the capacitance and the charging resistance. You can either enter a resistance value and calculate the corresponding time period or calculate the resistance that you must use to hold the circuit for a given time in the unstable state (depending on the selection done in the Options menu). Enter the circuit values and push the Calculate button. Then push the SET switch. The output of the circuit becomes high (LED2 goes on) and stays high until the time period is over. Note, that after the switch has been pushed, this process is started and goes its way; what I mean is that pushing the switch during the unstable state has no influence on the behavior of the circuit (duration of the unstable state). The application has however an option to add a RESET switch to the circuit; pushing it, immediately resets the circuit to its stable state.
The described timer circuits are only functioning correctly with resistance values being in a certain interval (1kΩ to 1MΩ). The application accepts values between 100Ω and 10MΩ. If you enter values outside this range, you get an error message. This is also true, if you enter a time period that would make necessary a resistance outside the range. The application only accepts standard resistor values. Entering a non-standard value results in an error message. If the resistance has to be calculated, the given time period is automatically adjusted (display of a warning message, unless warnings are set to be suppressed) to get a resistance with standard value.
It is evident that if the time period is very small, it will not be possible to follow the process and that below a certain value the application will not be fast enough to work correctly. The program doesn't check the time period value; when ensuring yourself that its value is greater than 100ms, there shouldn't be a problem.
Astable circuits.
Astable circuits have no stable state, what means that they alternate automatically between two unstable states, being in an "active" state during a given mark time and being in an "inactive" state during a given space time (the time period in this case being the sum of the mark time and the space time). Mark time and space time depend on the time it takes to charge resp. discharge the capacitor. Thus the mark time depends on the capacitance and the resistances R1 and R2 and the space time depends on the capacitance and the resistance R1. This means that (with the basic implementation of the circuit), the mark time is always greater than the space space time and the duty cycle is always greater than 50%. There is however a simple way to to be able to get duty cycles less than 50%: Connecting a diode in parallel with R2 (this implementation of the circuit may be selected in the Options menu), this resistance is bypassed during the capacitor's charge and you can get any duty cycle you want (in particular exactly 50% with R1 = R2).
You can either enter the two resistance values and calculate the corresponding mark and space time or calculate the resistances that you must use to get a given mark and space time (depending on the selection done in the Options menu). Enter the circuit values and push the Calculate button. Then push Start to start the simulation. The output of the circuit becomes high (LED2 on) and stays high during the mark time, then becomes low (LED2 off) and stays low during the space time. The actual values of mark and space time are displayed as the simulation proceeds.
The remarks made for the monostable resistance also apply to the resistances of the astable. Another error message that you may get with this circuit concerns the input of time values resulting in a duty cycle greater than 50% with a circuit implementation without diode.
If the time period is very small, it will be impossible to follow the flickering of the LED. And if it is less than a certain value, the application will not be fast enough to work correctly. The program doesn't check the mark and space time values; when ensuring yourself that these values are greater than 100ms, there shouldn't be a problem.