555 timer IC - Calculating resistance needed

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Discussion Overview

The discussion revolves around calculating the resistance needed for a 555 timer IC in an astable oscillator configuration. Participants are exploring how to properly charge and discharge a capacitor to prevent an LED from blowing, while addressing issues related to circuit resistance and LED brightness. The conversation includes technical calculations, component specifications, and the importance of referring to datasheets.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant notes that the current circuit has too much resistance, resulting in a dim LED, and seeks to calculate appropriate resistor values for R_A and R_B.
  • Another participant recalls that the 555 timer's internal comparator voltages are derived from a resistor string, suggesting that the switching levels for a 9V supply would be 3V and 6V.
  • Concerns are raised about whether the participant is using the design equations from the 555 datasheet or attempting to derive values independently.
  • A participant mentions that the LED is in series with a 470Ω resistor, calculating the current through the LED based on the voltage drop across it, and suggests that the average current may explain the dimness.
  • There is a suggestion to either reduce the series resistor to increase brightness or adjust the ON/OFF ratio to achieve a brighter LED while maintaining safety.
  • Another participant emphasizes the importance of consulting the datasheet for accurate information and calculations.

Areas of Agreement / Disagreement

Participants express differing views on the approach to calculating resistance and the importance of the datasheet. There is no consensus on the best method for achieving the desired LED brightness or the correct resistor values.

Contextual Notes

Some participants highlight the need for clarity on the assumptions regarding voltage levels and the relationship between resistance, capacitance, and LED performance. The discussion reflects uncertainty about the calculations and the implications of component specifications.

Who May Find This Useful

Individuals interested in electronics, particularly those working with 555 timer circuits, LED applications, and astable oscillators, may find this discussion relevant.

ToXic_Knight
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Voltage = 9.0 V, Current = 20 mA, Capacitor (C) = 10 microF, Resistance before (too high) [R_A = R_B] = 10 k Ohms.

In a a-stable oscillator, calculating the resistance needed for two resistors (R_A = R_B) to discharge and charge a capacitor (C) so that a LED does not blow. Currently my circuit has much too much resistance for testing and it works with the LED very dimly.

Here is the worksheet: http://i.imgur.com/nx6bnkw.jpg

a-stable oscillator: http://i.imgur.com/xWSC25R.jpg

Relevant equations
I know questions I have to do shortly require a few calculations with the resistance and capacitor, but I need other values first.
t_charge = (R_A + R_B)C sec

The attempt at a solution
I assume V_cc is the input voltage, so that is 9V. I have measured that the amplitude from V_low to V_high is 9V. I thought V_low should be zero, but the diagram has it separate and I don't know whether to follow that or not.

I know that V_D to V_U has amplitude 3V, but I have no idea what the actual values are.

This is probably very wrong, but I could assume that the distance between zero and V_D and V_CC is 9-3 = 6 V and that each then must be 3 V.

Time to charge is 150 ms, time to discharge is 65 ms.

Summary of measurements: http://puu.sh/hvONo/851b30c03f.png

I'll keep editing this post as I work on it, but I've been stuck for days without making much progress.
Other questions:
Reading the questions ahead, I don't feel very confident with them either, so some tips or telling me what equations/variables to use would be very helpful.

Scary questions: http://i.imgur.com/Mf6iBij.jpg
http://i.imgur.com/2jSn6A9.jpg
http://i.imgur.com/0S8WBj8.jpg
 
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From memory, doesn't the datasheet for the 555 indicate that the internal comparator voltages VD and VU are derived from a string of 3 equal-valued resistors between VCC and ground? So for VCC of 9v, these switching levels you see on the capacitor curve will be 3V and 6V, resp.

Are you determining the charging/discharging durations using the design equations provided in this chip's datasheet, or are you attempting to derive these for yourself from the exponential relationship?

Do you have the 555 datasheet (probably about 15 pages) as a pdf that you can constantly refer to?

ToXic_Knight said:
I know that V_D to V_U has amplitude 3V, but I have no idea what the actual values are.
This implies that you are attempting this lab investigation without looking at the datasheet ...
kPdWZ.png


If that is the case, I don't think we can help you.

P.S. your thread title was uninformative, I edited it
 
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ToXic_Knight said:
In a a-stable oscillator, calculating the resistance needed for two resistors (R_A = R_B) to discharge and charge a capacitor (C) so that a LED does not blow. .

There is a 470R in series with the LED. If the LED was ON all the time the current in the LED would be about... (9-2)/470 = 15mA (Assuming the voltage drop across the diode is about 2V). Many LED will work just fine at 20mA so the circuit might be fail safe (eg LED won't blow even if the oscillator stops in the ON state). What does the data sheet for your LED say?

Currently my circuit has much too much resistance for testing and it works with the LED very dimly

Your "summary of measurements" sheet suggests the ON:OFF ratio is 75:150 so the LED is ON about 75/225 or 33% of the time. That drops the average LED current to about 0.33 * 15mA = 5mA. May explain why it's dim?

At this point I have to ask... What are you trying to achieve?

You could reduce the 470R to make the LED brighter but this might stop it being fail safe.

You could increase the ON/OFF ratio to make the LED brighter. The data sheet for the 555 explains how to calculate the ON/OFF ratio. See 7.4.2. pages 10 and 11.

http://www.ti.com/lit/ds/symlink/lm555.pdf
 
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NascentOxygen said:
From memory, doesn't the datasheet for the 555 indicate that the internal comparator voltages VD and VU are derived from a string of 3 equal-valued resistors between VCC and ground? So for VCC of 9v, these switching levels you see on the capacitor curve will be 3V and 6V, resp.

Are you determining the charging/discharging durations using the design equations provided in this chip's datasheet, or are you attempting to derive these for yourself from the exponential relationship?

Do you have the 555 datasheet (probably about 15 pages) as a pdf that you can constantly refer to?This implies that you are attempting this lab investigation without looking at the datasheet ...
kPdWZ.png


If that is the case, I don't think we can help you.

P.S. your thread title was uninformative, I edited it

ToXic_Knight said:
Thanks, sorry, I'm not used to this and keep forgetting to check data sheets and other information, thanks.
Thanks, sorry, I'm not used to this and keep forgetting to check data sheets and other information, thanks.
 

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