555 and op amp driving 8 ohm speaker - Page 1
Forum section: electronics / projects

author: sentry7
author_rank: Regular Contributor
timestamp: April 17, 2015, 03:48:37 am
content: Hey Guys,I was playing with oscillators and built these two circuits. One is based on a 555, and the other is a relaxation oscillator. The 555 circuit was much louder than the square wave produced by the op amp.I understand that the 741 op amp is not a good choice to drive an 8 ohm impedance as it cannot supply enough output current. When I looked at the datasheets for the 555 and the op amp, their maximum output currents were 15mA and 25mA, respectively. How is it that the 555 produced higher power waveforms when its max output current is less than the 741?NOTE: These are two separate circuits.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 17, 2015, 03:51:35 am
content: Quote from: sentry7 on April 17, 2015, 03:48:37 amYou were looking at the wrong datasheet, or the wrong property -- the 555 output pin is capable of over 100mA.This circuit needs an LM386, to keep in the spirit of the vintage parts.Since it's a square wave, you could also use some BJT transistors, such as an emitter follower ("class C complementary pair" style), to amplify and match to the low impedance.Tim

author: sentry7
author_rank: Regular Contributor
timestamp: April 17, 2015, 03:59:39 am
content: Quote from: T3sl4co1l on April 17, 2015, 03:51:35 amWrong property, yes. I was confused about these two. Why is it called "sink" current? Wouldn't it be the current that's being "sourced" to other devices?

author: matseng
author_rank: Frequent Contributor
timestamp: April 17, 2015, 04:21:23 am
content: Hotlinking an image fromhttp://www.555-timer-circuits.com/using-the-output.htmlexplaining sourcing and sinking..

author: sentry7
author_rank: Regular Contributor
timestamp: April 17, 2015, 04:36:11 am
content: Quote from: matseng on April 17, 2015, 04:21:23 amSo the 555 is sourcing the speaker, right? Wouldn't this have the maximum of 15mA?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 17, 2015, 04:54:07 am
content: Where's that come from??This, from the horse's mouth (TI/National's LM version),http://www.ti.com/lit/ds/symlink/lm555.pdfHeadlined, "Output Can Source or Sink 200mA"Data:Output Voltage Drop (Low), Isink = 200mA, 2.5V typ.Output Voltage Drop (High), Isource = 200mA, 12.5V typ.(Note that both "drops" are measured with respect to ground. Preferably, sourcing should be measured relative to VCC, but at 15V supply, it's clear that it's also a 2.5V drop.)(They don't give the symbols for these parameters, but they're usually called V_OL and V_OH respectively, and the currents, I_OL and I_OH. This notation is most common for logic chips. At 5V supply, the 555 is pretty much TTL compatible, and not bad for CD4000 series CMOS for supply voltages above 6 or 8V.)Graphs:Fig. 3 V_OH vs. I_OH (note this *is* in terms of drop from VCC)Fig. 4 V_OL vs. I_OLWait a minute... colors? Scope shots? Layouts?! Holy crapamoli, they updated this datasheet! As recently as January, no less! Although... they specify LMC555 on the layout, which is unfortunately a different animal. That's weird.Beware that there are a great many versions of the "555"... from the original NE555 (or was it the uA555?? I forget) to the LM and other versions. And the CMOS variants, including NE7555, LMC555 and etc. Which generally support much lower current levels (e.g., V_OH = 10.5V min. at 12V supply, 10mA load).Tim

author: Zero999
author_rank: Super Contributor
timestamp: April 17, 2015, 08:30:03 am
content: The speaker is AC coupled via C3 so the 555 is acting as a push-pull stage, both sourcing and sinking current.R4 (555 circuit) and R6 (741 circuit) don't do anything useful and should be removed.The 555 may get too hot if it's lefty driving a speaker for too long. The 741 will current limit the current to below 40mA which is why it's not as loud.

author: sentry7
author_rank: Regular Contributor
timestamp: April 17, 2015, 01:33:39 pm
content: Quote from: Hero999 on April 17, 2015, 08:30:03 amAh, thanks; this helps clean up the design. I thought that whenever you do capacitive coupling, you put down a shunt resistor to limit the current supplied to the load? Maybe I'm thinking of something else...T3sl4co1l,Will get back to you later with the exact datasheet.

author: matseng
author_rank: Frequent Contributor
timestamp: April 17, 2015, 01:44:20 pm
content: Isn't R1 also unnecessary? Why have a pull up at the output of a 555 that have a push-pull output stage?

author: Zero999
author_rank: Super Contributor
timestamp: April 18, 2015, 09:49:01 pm
content: Yes R1 is unnecessary on the 555 timer circuit.Want it to be really loud? Add an h-bridge.