# New Old Guy

#### ScottWang

Joined Aug 23, 2012
6,926
Yeabut the drawing in post #12 is a, "Ground to start the timer" circuit. The monostable naturally uses a ground for a trigger. I think you are chasing the wild goose.
Chasing the wild goose and no airplane to catch up ...

#### ian field

Joined Oct 27, 2012
6,539
All resistors are using 1/4 watt
D1 - 1N4007
VR1 - potentiometer,pot or VR.
C1 - Electrolytic capacitor, it has ±20% tolerance
C2 = 0.01 mylar capacitor
T=1.1(R2+VR1)*C1 = 11 sec
You can using VR1=100K and C1=100uf/25V or VR1=1MΩ and C1=10uf/25V.
LED1 = 2V/20mA/3mm
R4 = 1.2K
You can using LED1 = 2V/10mA/3mm, R4=1.2K or LED1 = 3V/20mA/5mm, R4 = 560Ω.
If the relay turn on, but it can't keep stable then you can reducing the R3 from 10K to 8.2K.
Sw1 - push button switch.
Relay - 12V, Com = common pin, N.O = Normal Open, N.C = Normal Close
Normally we using Com and N.O two pins to control the device.

View attachment 83830
Not sure if it applies to that exact configuration, but sometimes its as well to delay the reset pin with a C/R charging network.

Whether its needed or not, it ensures output in the reset state at the instant of switch on.

The C/R delay only needs to be a few milliseconds.

#### Switched

Joined Apr 9, 2015
16
ScottWang, #12,

you are chasing the wild goose
I thought that it was a chicken.

Ron
All resistors are using 1/4 watt
D1 - 1N4007
VR1 - potentiometer,pot or VR.
C1 - Electrolytic capacitor, it has ±20% tolerance
C2 = 0.01 mylar capacitor
T=1.1(R2+VR1)*C1 = 11 sec
You can using VR1=100K and C1=100uf/25V or VR1=1MΩ and C1=10uf/25V.
LED1 = 2V/20mA/3mm
R4 = 1.2K
You can using LED1 = 2V/10mA/3mm, R4=1.2K or LED1 = 3V/20mA/5mm, R4 = 560Ω.
If the relay turn on, but it can't keep stable then you can reducing the R3 from 10K to 8.2K.
Sw1 - push button switch.
Relay - 12V, Com = common pin, N.O = Normal Open, N.C = Normal Close
Normally we using Com and N.O two pins to control the device.

View attachment 83830
Scott,

I'm absorbing what you have given to me. You are using the NPN 2N3904 to convert V+ to 0V. Right? (inverse of the PNP 2N3906 circuit that I posted.) I will never be an electronic wizard but, I will die trying to be one.

I actually need V+ output for my circuit. I have a chain of events and if they all complete prior to timer expiring then the V+ will facilitate initializing the circuit. Else I will derive 12V+ 15A from another relay in the circuit.

Ron

#### ScottWang

Joined Aug 23, 2012
6,926
ScottWang, #12,

I thought that it was a chicken.

Ron

Scott,

I'm absorbing what you have given to me. You are using the NPN 2N3904 to convert V+ to 0V. Right? (inverse of the PNP 2N3906 circuit that I posted.) I will never be an electronic wizard but, I will die trying to be one.

I actually need V+ output for my circuit. I have a chain of events and if they all complete prior to timer expiring then the V+ will facilitate initializing the circuit. Else I will derive 12V+ 15A from another relay in the circuit.

Ron
Chicken is easy to catch, but the choose, maybe it will chasing us ...

The posted circuit in #12 that it needs two contacts as a high level and gnd to connect the pin 2 from high to low.

You said that you need a high Vout, do you mean that the relay part or the part of trigger input pin 2?

#### Switched

Joined Apr 9, 2015
16
Chicken is easy to catch, but the choose, maybe it will chasing us ...

The posted circuit in #12 that it needs two contacts as a high level and gnd to connect the pin 2 from high to low.

You said that you need a high Vout, do you mean that the relay part or the part of trigger input pin 2?
Scott,

Ground signal is consistent with the signal emitted by my application. The timer relay is part of the logic in the circuit so it does not handle high amperage.

Please confirm my understanding of your use of the NPN 2N3904 transistor so that I can add it to my notebook...... if I'm correct.

You are using the NPN 2N3904 to convert V+ to 0V. Right?

Ron

#### #12

Joined Nov 30, 2010
18,210
The only 3904 I see is a relay driver. When the timer output goes high, the 3904 collector goes low and the relay clicks. Please point to a drawing to accompany your questions.

#### Switched

Joined Apr 9, 2015
16
#12,

My question was posed to Scott, however, since you chose to reply then let's parse your comment.

The only 3904 I see is a relay driver: there is only one 2N3904 transistor in Scott's schematic so why would I need to provide you with a drawing?

When the timer output goes high: is that not positive current?

the 3904 collector goes low and the relay clicks: does not that require a ground for the relay induction coil?

So is not the 2N3904 transistor inverting the positive current to a ground for the relay induction coil?

Ron

#### ScottWang

Joined Aug 23, 2012
6,926
Scott,

Ground signal is consistent with the signal emitted by my application. The timer relay is part of the logic in the circuit so it does not handle high amperage.

Please confirm my understanding of your use of the NPN 2N3904 transistor so that I can add it to my notebook...... if I'm correct.

You are using the NPN 2N3904 to convert V+ to 0V. Right?

Ron
For a logical explanation:
1. When the timer is counting, the output pin 3 of 555 is goes low, the 2N2904 of Vbe is turn off, and the Vce is also turn off, the c is high, so the relay turn off, there is no any current flows through the coil of relay.

2. When the timer is time up, the output pin 3 of 555 is goes high, the 2N2904 of Vbe is turn on, and the Vce is also turn on, the c is low, so the relay turn on, the current flows through the coil of relay from +12V → (+)relay(-) → c of 2N3904 is a low level as ground → e is ground.

So, is that clear enough for you, or you need some more?

#### Switched

Joined Apr 9, 2015
16
For a logical explanation:
1. When the timer is counting, the output pin 3 of 555 is goes low, the 2N2904 of Vbe is turn off, and the Vce is also turn off, the c is high, so the relay turn off, there is no any current flows through the coil of relay.

2. When the timer is time up, the output pin 3 of 555 is goes high, the 2N2904 of Vbe is turn on, and the Vce is also turn on, the c is low, so the relay turn on, the current flows through the coil of relay from +12V → (+)relay(-) → c of 2N3904 is a low level as ground → e is ground.

So, is that clear enough for you, or you need some more?
Scott,

Thank you for the in depth explanation.

Ron

#### Switched

Joined Apr 9, 2015
16
Scott,

Regarding omitting the capacitor on pin #5 or the voltage control. The following was copied from the TI NE555p data sheet.

9.2.2 Pulse-Width Modulation

The operation of the timer CAN BE modified by modulating the internal threshold and trigger voltages, which is accomplished by applying an external voltage (or current) to CONT. Figure 18 shows a circuit for pulse-width modulation. A continuous input pulse train triggers the monostable circuit, and a control signal modulates the threshold voltage. Figure 19 shows the resulting output pulse-width modulation. While a sine-wave modulation signal is shown, any wave shape could be used.

CAN BE
implies that it is optional, however,

NOTE A: The modulating signal can be direct or capacitively coupled
to CONT. For direct coupling, the effects of modulation
source voltage and impedance on the bias of the timer
should be considered.

I knew that the purpose of the .01uF capacitor is to generate a PWM signal at the voltage control pin. In 9.2.2 Pulse-Width Modulation it states that analog voltage is another option, however, I cannot find any where it advocates for leaving pin # 5 open. As I understand the purpose of the voltage control is to regulate the timer voltage to 2/3 of the Vcc.

I have been told that I should omit the .01uF capacitor at pin #5 due to my application and the inherent noise. I have found a circuit where they used 10k resistance between pin #5 and ground while using the .01uF capacitor. I recognize that as being a noise filter.

I'm not questioning what I have been told because the source is very knowledgeable, however I would like a second opinion based on the above or factors that I have not considered.

I would also like to learn why your schematic flips the output to ground as opposed to using the positive current to energize the high amperage switching relay. My understanding is that the positive output current is not substantial enough so you use the Vcc to energize the high amperage switching relay.

Ron

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#### ScottWang

Joined Aug 23, 2012
6,926
Scott,

Regarding omitting the capacitor on pin #5 or the voltage control. The following was copied from the TI NE555p data sheet.

9.2.2 Pulse-Width Modulation

The operation of the timer CAN BE modified by modulating the internal threshold and trigger voltages, which is accomplished by applying an external voltage (or current) to CONT. Figure 18 shows a circuit for pulse-width modulation. A continuous input pulse train triggers the monostable circuit, and a control signal modulates the threshold voltage. Figure 19 shows the resulting output pulse-width modulation. While a sine-wave modulation signal is shown, any wave shape could be used.

CAN BE
implies that it is optional, however,

NOTE A: The modulating signal can be direct or capacitively coupled
to CONT. For direct coupling, the effects of modulation
source voltage and impedance on the bias of the timer
should be considered.
It is talking about if you want to use pwm then adds or not adds a capacitor will affecting the DC current of 555 itself, If you don't need the pwm function then you could ignore it, and you just connecting a 0.01 uf capacitor to ground.

I knew that the purpose of the .01uF capacitor is to generate a PWM signal at the voltage control pin. In 9.2.2 Pulse-Width Modulation it states that analog voltage is another option, however, I cannot find any where it advocates for leaving pin # 5 open. As I understand the purpose of the voltage control is to regulate the timer voltage to 2/3 of the Vcc.
No, it's wrong.
Why the pin 5 has such 1/3,2/3 Vcc these things, because that is the internal structure used three 5K resistors to provided the fixed reference voltage to compared with pin 2(-) as 1/3 Vcc and pin 6(+) as 2/3 Vcc, and that is why the pin 5 can do the pwm function, you can see the internal structures of 555 about the input sections.

I have been told that I should omit the .01uF capacitor at pin #5 due to my application and the inherent noise. I have found a circuit where they used 10k resistance between pin #5 and ground while using the .01uF capacitor. I recognize that as being a noise filter.
0.01 uf is the filter, but the 10K will be changed the internal reference voltages from 2/3 Vcc to 1/2 Vcc, why is that , because the upper resistor is still 5K and the lower resistor became (5K+5K)//10K, the meaning of these are 5K in series with 5K is equal to 10K, and 10K in parallel with 10K is 5K, the upper is 5K and lower is 5K, so the pin 5 became 1/2 Vcc, note here, when you changing the voltage of pin 5 will change the output of comparing and it will also change the out of pin 3, that's what the pwm doing, using the external voltage or signal to change the pin 5 will caused the pin 3 output changed.

I'm not questioning what I have been told because the source is very knowledgeable, however I would like a second opinion based on the above or factors that I have not considered.

I would also like to learn why your schematic flips the output to ground as opposed to using the positive current to energize the high amperage switching relay. My understanding is that the positive output current is not substantial enough so you use the Vcc to energize the high amperage switching relay.

Ron
If you are talking about why I added a bjt to drive the relay, the big reason is that the power consumption considering, although 555 provided 200 mA, the current is enough for many relays, but considering the chip itself maybe it can't afford the heat, so I added a bjt to do the job, of course, sometimes we will also afraid of the current is not enough, but mostly the reason is the heat, you can decide that according to how much current the relay will draw.

#### Switched

Joined Apr 9, 2015
16
Scott,

My grand father always told me that men that were to smart to ask questions were often the dumbest men. I would rather admit or show my ignorance then to operate under a misconception. Thank you for your explanation.

BTW, what is the purpose for the 1k resistor in front of the VR?

Ron

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